Recognize Percent Growth or Decay
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Algebra 2 › Recognize Percent Growth or Decay
A company tracks the number of active users each month:
Month: 0, 1, 2, 3
Users: 12,000; 13,200; 14,520; 15,972
Does this represent constant percent change, and if so, what is the percent growth rate per month?
No; it’s linear because it increases by 1,200 each month
Yes; 12% growth per month (growth factor $b=1.12$)
Yes; 1.1% growth per month (because ratios are 1.1)
Yes; 10% growth per month (growth factor $b=1.10$)
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Applying the ratio test: 13,200/12,000 = 1.1, 14,520/13,200 = 1.1, 15,972/14,520 = 1.1, so constant ratios at 1.1 confirm exponential growth with b=1.10, and percent rate (1.10 - 1)*100% = 10% per month. Choice C correctly identifies yes, 10% growth per month through these constant ratios and rate calculation. A distractor like choice A might confuse it with linear due to increasing values, but the differences (1,200; 1,320; 1,452) aren't constant, so it's not linear—nice work distinguishing them! The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A laptop loses value each year due to depreciation. Its value is $1200 after 0 years, $1020 after 1 year, $867 after 2 years, and $736.95 after 3 years.
From the data, what is the percent rate of change per year, and is it growth or decay?
15% growth per year
15% decay per year
0.85% decay per year
10% growth per year
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Let's calculate the ratios: 1020/1200 = 0.85, 867/1020 = 0.85, 736.95/867 = 0.85. All ratios equal 0.85, confirming exponential decay with base 0.85. Choice B correctly identifies 15% decay per year through constant ratios: base 0.85 means each year retains 85% of previous value, losing 15% (100% - 85% = 15%). Choice C incorrectly claims 15% growth—but ratios less than 1 indicate decay, not growth! The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A recycling bin contains 500 pounds of material. Each day, 6% of the material is removed. Which statement correctly identifies the constant percent rate and the exponential model for the amount remaining after $d$ days?
Linear decay; $M(d)=500-0.06d$
6% decay; $M(d)=500(0.94)^d$
94% decay; $M(d)=500(0.06)^d$
6% growth; $M(d)=500(1.06)^d$
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! If 6% is removed each day, then 94% remains (100% - 6% = 94%). This means we multiply by 0.94 each day: Day 0: 500 pounds, Day 1: 500 × 0.94, Day 2: 500 × (0.94)², giving us M(d) = 500(0.94)^d. Choice B correctly identifies 6% decay with the model M(d) = 500(0.94)^d (base 0.94 represents 94% remaining). Choice A incorrectly suggests growth when material is being removed; Choice C misinterprets the situation as 94% decay (which would leave only 6%); Choice D suggests a linear model which doesn't match percent removal. The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
Two savings plans are described below.
Plan 1: Start with $2000 and add $50 each month.
Plan 2: Start with $2000 and increase the balance by 2% each month.
Which plan shows constant percent change (exponential), and which shows constant additive change (linear)?
Plan 1 is exponential; Plan 2 is linear
Both plans are exponential because both increase each month
Plan 2 is exponential; Plan 1 is linear
Both plans are linear because they both change monthly
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Plan 1 adds $50 each month—this is constant additive change, making it linear. Plan 2 increases the balance by 2% each month, meaning each month's balance is 102% of the previous (multiply by 1.02)—this is constant percent change, making it exponential. Choice D correctly identifies Plan 2 as exponential (constant percent change) and Plan 1 as linear (constant additive change). Choice A reverses these classifications, missing that "add $50" signals linear while "increase by 2%" signals exponential. The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms!
A medication amount in the bloodstream is modeled by $M(t)=80(0.92)^t$, where $t$ is in hours.
What percent does the amount change each hour, and is it growth or decay?
92% decay per hour
8% decay per hour
8% growth per hour
0.92% decay per hour
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! From the function M(t) = 80(0.92)^t, the base is 0.92, indicating a constant multiplicative factor less than 1 each hour. Choice A correctly identifies 8% decay per hour through the base analysis (0.92 = 1 - 0.08). A distractor like choice B misinterprets the base as the percent decay directly, but it's the retention factor—remember, percent decay is 1 - base! The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
Two phone plans track the number of users over months.
Plan A adds 250 users each month.
Plan B increases the number of users by 5% each month.
Which statement is correct about the type of change for each plan?
Plan A is exponential; Plan B is linear
Both plans are linear because they change each month
Both plans are exponential because they both increase
Plan A is linear (constant additive); Plan B is exponential (constant percent)
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Plan A adds 250 users each month—this is a constant amount added, making it linear (if starting at N₀, then N(t) = N₀ + 250t). Plan B increases by 5% each month—this is a constant percent change, making it exponential (if starting at N₀, then N(t) = N₀(1.05)^t). Choice C correctly identifies Plan A as linear (constant additive change of +250) and Plan B as exponential (constant percent change of 5%). Choice A reverses the classifications; Choice B incorrectly claims both are linear; Choice D incorrectly claims both are exponential just because they increase. The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A company’s number of subscribers changes as shown:
Week: 0, 1, 2, 3
Subscribers: 1000, 1080, 1166.4, 1259.712
Classify the pattern and identify the constant percent rate per week.
Linear growth; increases by 80 subscribers per week
Exponential growth at 1.08% per week
Exponential decay at 8% per week
Exponential growth at 8% per week
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Let's calculate the ratios: 1080/1000 = 1.08, 1166.4/1080 = 1.08, 1259.712/1166.4 = 1.08. All ratios equal 1.08, confirming exponential growth with base 1.08. Choice C correctly identifies exponential growth at 8% per week through constant ratios: base 1.08 means multiplying by 1.08 each week, which is 108% of previous value, representing 8% growth (108% - 100% = 8%). Choice A incorrectly claims linear growth of 80 subscribers, but checking differences: 1080 - 1000 = 80, 1166.4 - 1080 = 86.4—the differences aren't constant! The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A scientist records a sample’s mass (in grams) after each processing step:
Step $n$: 0, 1, 2, 3
Mass $M(n)$: 200, 180, 162, 145.8
Classify the pattern and identify the constant percent rate per step.
Neither; it is linear because it decreases by 20 grams each step.
Exponential decay at 10% per step.
Exponential decay at 0.90% per step.
Exponential growth at 10% per step.
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval.
Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor!
To check, compute the ratios from the table: 180/200 = 0.9, 162/180 = 0.9, and 145.8/162 = 0.9, showing a constant ratio of 0.9, fitting exponential decay.
Choice C correctly identifies exponential decay at 10% per step through constant ratios of 0.9 (since 1 - 0.9 = 0.1 or 10%).
One distractor suggests linear by 20 grams, but differences are 20, 18, 16.2—not constant; excellent work spotting the ratio pattern instead!
The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably!
Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A city’s population is recorded every 5 years:
Year: 0, 5, 10, 15
Population: 50,000; 53,000; 56,000; 59,000
Classify the pattern as exponential growth, exponential decay, or neither. (Hint: compare differences vs ratios.)
Exponential growth, because the ratio is constant.
Exponential decay, because the population increases by smaller amounts each interval.
Exponential growth at 6% per 5 years.
Neither; it shows approximately constant additive change (more linear than exponential).
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval.
Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor!
Differences are +3000 each (constant, linear); ratios are 53000/50000=1.06, 56000/53000≈1.0566, 59000/56000≈1.0536—not constant, so not exponential.
Choice C correctly identifies neither (more linear than exponential) through comparing constant differences vs. non-constant ratios.
One distractor suggests exponential growth because it increases, but ratios aren't constant—nice try, but always verify both tests!
The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably!
Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms! Check both: if differences constant → linear. If ratios constant → exponential. Usually only one pattern holds. The language helps too: 'grows by $50 per year' = linear (additive), 'grows by 5% per year' = exponential (multiplicative)!
A radioactive sample’s mass is recorded every day:
Day: 0, 1, 2, 3
Mass (g): 50, 46, 42, 38
Does this represent constant percent decay? (Hint: constant percent change means a constant ratio, not a constant difference.)
Yes; 4% decay per day because it decreases by 4 each day
No; the ratios $46/50$, $42/46$, and $38/42$ are not all equal
Yes; it is exponential decay because it decreases by 4 g each day
Yes; 8% decay per day because $46/50=0.92$
Explanation
This question tests your ability to recognize exponential growth or decay—situations where a quantity changes by a constant percent (not constant amount) per time interval. Constant percent growth means multiplying by the same factor greater than 1 each interval: if something grows 5% per year, each year's value is 105% of the previous (multiply by 1.05). Constant percent decay means multiplying by a factor between 0 and 1: if something decreases 15% per year, each year retains 85% of previous (multiply by 0.85). The key test: calculate ratios of consecutive values. If ratios are constant, it's exponential with that ratio as the growth/decay factor! Let's check the ratios: 46/50 = 0.92, 42/46 ≈ 0.913, 38/42 ≈ 0.905. The ratios are not constant (0.92 ≠ 0.913 ≠ 0.905), so this is NOT exponential decay. Instead, the differences are constant: 50-46=4, 46-42=4, 42-38=4, indicating linear decay. Choice B correctly identifies that the ratios are not all equal, confirming this is not constant percent decay. Choice C incorrectly claims exponential decay based on the constant difference of 4g, which actually indicates linear change. The ratio test for exponential: (1) from table, divide consecutive y-values: y₂/y₁, y₃/y₂, y₄/y₃, (2) if all equal → exponential with that ratio as base b, (3) if b > 1 → growth; if 0 < b < 1 → decay, (4) calculate percent rate: r = b - 1, convert to percent. Example: ratios all 1.06 → base 1.06 → growth → rate = 1.06 - 1 = 0.06 = 6% per interval. This systematic check identifies exponential patterns reliably! Don't confuse exponential with linear: linear adds the same amount each time (constant differences like +50, +50, +50), exponential multiplies by same factor (constant ratios like ×1.1, ×1.1, ×1.1). Both are patterns of regular change, but different mechanisms!