Linear & Exponential Growth

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SAT Math › Linear & Exponential Growth

Questions 1 - 10
1

A taxi charges a base fare plus a per-mile fee. The relationship between miles $x$ and total cost $y$ is $y=2.75x+4.50$. What does the slope represent, and what is the total cost for a 6-mile ride?​

$4.50$ per mile; $21.00$

$4.50$ base fare; $18.00$

$2.75$ base fare; $21.00$

$2.75$ per mile; $21.00$

Explanation

The question provides the taxi fare equation y = 2.75x + 4.50 and asks for the meaning of the slope and the cost of a 6-mile ride. In this linear equation, the slope 2.75 represents the rate of change - the cost per mile ($2.75/mile). The y-intercept 4.50 is the base fare. For a 6-mile ride: y = 2.75(6) + 4.50 = 16.50 + 4.50 = 21.00 dollars. The key concept is interpreting slope as rate of change in context - here it's the variable cost per unit distance. Common errors include confusing the roles of slope and y-intercept in real-world applications.

2

A laboratory sample cools according to $T(t)=20+60(0.75)^t$, where $T$ is temperature in °C and $t$ is time in minutes. What does the 20 represent in context, and what is $T(2)$ to the nearest tenth?​

Cooling rate; 63.8°C

Ambient temp; 63.8°C

Initial temp; 53.8°C

Ambient temp; 53.8°C

Explanation

The question asks about the cooling model T(t) = 20 + $60(0.75)^t$ and what the constant 20 represents. In Newton's Law of Cooling models, the equation typically has the form T = T_ambient + (T_initial - T_ambient)(decay $factor)^t$. Here, 20 represents the ambient (room) temperature that the sample approaches as t increases. To find T(2): T(2) = 20 + $60(0.75)^2$ = 20 + 60(0.5625) = 20 + 33.75 = 53.75°C, which rounds to 53.8°C. The key insight is recognizing that exponential decay models often include a horizontal asymptote representing the limiting value.

3

A water tank contains 300 liters and is being drained at a constant rate of 12 liters per minute. Which equation gives the amount of water $W(t)$ after $t$ minutes, and how much water remains after 15 minutes?​

$W=12t-300$; 120 L

$W=300-15t$; 75 L

$W=300-12t$; 120 L

$W=300(0.96)^t$; 240 L

Explanation

The question involves a water tank draining at a constant rate, which represents linear decrease. Starting with 300 liters and draining 12 liters per minute gives the equation W(t) = 300 - 12t. This follows the linear model y = mx + b where the negative slope (-12) represents the drainage rate. After 15 minutes: W(15) = 300 - 12(15) = 300 - 180 = 120 liters. The key concept is that constant rate problems result in linear models, not exponential ones. A common error would be using an exponential decay model when the problem states a constant rate.

4

A bacteria culture starts with 200 cells and is modeled by the equation $N(t)=200(1.5)^t$, where $t$ is the number of hours after the start. Which statement best interprets the parameter $1.5$ in this context, and what does it imply about the type of growth?​

Adds 50% cells/hr; linear growth

Multiplies by 50 each hr; exponential growth

Multiplies by 1.5 each hr; exponential growth

Adds 1.5 cells/hr; linear growth

Explanation

The question asks for the interpretation of the parameter 1.5 in the exponential equation N(t) = $200(1.5)^t$. This equation follows the form of exponential growth where the base (1.5) represents the growth factor. In exponential models, the population is multiplied by the growth factor each time period, so the bacteria population multiplies by 1.5 each hour. This means the population increases by 50% each hour (since 1.5 = 1 + 0.50). The key error would be confusing multiplicative growth with additive growth - exponential functions multiply by a constant factor, not add a constant amount.

5

A student compares two investment options starting at $\$1{,}000$. Option L grows linearly by adding $$60$ each year. Option E grows exponentially by multiplying by $1.05$ each year. After 10 years, which option has the greater value (nearest dollar), and what is that value?​

Option L; $1,600$

Option L; $1,629$

Option E; $1,629$

Option E; $1,600$

Explanation

The question compares linear growth (Option L: add $60/year) versus exponential growth (Option E: multiply by 1.05/year) over 10 years, both starting at $1,000. For Option L: 1000 + 60(10) = 1000 + 600 = $1,600. For Option E: 1000(1.05)^10 = 1000(1.6289) ≈ $1,629. Option E yields the greater value at $1,629. This illustrates that exponential growth, even with a modest 5% rate, eventually outpaces linear growth. The key insight for test-taking is that exponential growth compounds, making it more powerful over longer time periods even when the percentage seems small.

6

A streaming service’s subscribers follow the model $S=12000+800t$, where $t$ is months since launch. What does the number 800 represent in context?

Increase in subscribers per month

Initial subscribers at launch

Percent growth per month

Subscribers after 1 year

Explanation

This question tests interpretation of parameters in a linear model for subscriber growth. In S = 12000 + 800t, the coefficient 800 represents the rate of change - specifically, the increase in subscribers per month. The constant term 12000 is the initial number of subscribers at launch (when t = 0). Students often confuse these roles or misinterpret 800 as a percentage rather than an absolute increase. In linear models of the form y = mx + b, m always represents the rate of change per unit of the independent variable. When solving context problems, always identify what each parameter means in real-world terms.

7

A car’s value, $V$, depreciates by 18% each year. The value after $t$ years is modeled by $V=22000(0.82)^t$. What does the number $22000$ represent in this situation?

Value after 1 year

Initial value at $t=0$

Annual growth factor

Annual depreciation amount

Explanation

This question tests understanding of exponential decay model parameters, specifically what each number represents in context. In the equation V = $22000(0.82)^t$, the number 22000 is the coefficient that appears when t = 0, making it the initial value of the car. The base 0.82 represents the decay factor (1 - 0.18 = 0.82, since the car loses 18% of its value each year). A common mistake is thinking 22000 represents the value after one year, but that would be $22000(0.82)^1$ = 18040. When interpreting exponential models, always evaluate at t = 0 to find the initial value.

8

A savings account starts with $$\500$$ and grows by 6% each month. Which equation gives the balance $B$ after $m$ months (no additional deposits or withdrawals)?

$B=500(0.94)^m$

$B=500+1.06m$

$B=500+0.06m$

$B=500(1.06)^m$

Explanation

This problem asks for an exponential growth equation modeling compound interest. Since the account grows by 6% each month, the growth factor is 1 + 0.06 = 1.06, and we multiply the initial amount by this factor raised to the power of months: B = 500(1.06)^m. Linear models like B = 500 + 0.06m would only add 6% of the original $500 each month, not 6% of the current balance. The key distinction is that exponential growth compounds - each month's 6% is calculated on the new, larger balance. Always use exponential models for percentage-based growth or decay.

9

A rumor spreads so that the number of people who have heard it is multiplied by 1.25 each day. If 160 people have heard it on day 0, how many have heard it after 3 days?

250

312.5

320

460

Explanation

This calculation involves exponential growth over multiple time periods. Starting with 160 people and multiplying by 1.25 each day, after 3 days we have $160(1.25)^3$. Calculate $(1.25)^3$ = 1.953125, then 160 × 1.953125 = 312.5 people. This represents compound growth where each day's increase is 25% of the previous day's total, not the original amount. Students sometimes multiply 160 × 1.25 × 3 = 600, incorrectly treating it as simple interest. For exponential growth, always raise the growth factor to the power of time periods, then multiply by the initial value.

10

A lab culture doubles every 3 hours. At time $t=0$, there are 200 cells. Which equation models the number of cells $N$ after $t$ hours?

$N=200(2)^{t/3}$

$N=200+2(t/3)$

$N=200(2)^t$

$N=200(3)^{t/2}$

Explanation

This problem involves modeling exponential growth with a non-unit time factor. Since the culture doubles every 3 hours, after t hours it has gone through t/3 doubling periods. Starting with 200 cells, the model is N = $200(2)^(t/3$), which correctly accounts for the 3-hour doubling time. For example, after 6 hours (2 doubling periods), N = $200(2)^2 = 800$ cells. Students often write N = $200(2)^t$, forgetting to adjust for the doubling period. When the growth period doesn't match the time unit, always include the appropriate fraction in the exponent. Test your model at easy values like t = 3 to verify correctness.

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