Divide Unit Fractions by Whole Numbers
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5th Grade Math › Divide Unit Fractions by Whole Numbers
Maria has $$\frac{1}{3}$$ of a pizza that she wants to share equally among 4 friends. She calculates that each friend will get $$\frac{1}{12}$$ of the original pizza. To check her work, she multiplies $$\frac{1}{12} \times 4$$ and gets $$\frac{4}{12}$$. What should Maria do next to verify her answer is correct?
Simplify $$\frac{4}{12}$$ to $$\frac{1}{3}$$ and confirm it equals the amount she started with
Divide $$\frac{4}{12}$$ by 4 again to get $$\frac{1}{12}$$ and confirm this is her final answer
Add $$\frac{1}{12}$$ four times to get $$\frac{4}{48}$$ and check if this equals $$\frac{1}{3}$$
Convert $$\frac{1}{3}$$ to twelfths by multiplying both parts by 4 to get $$\frac{4}{12}$$
Explanation
To verify division, Maria should check that her answer times the divisor equals the dividend. She multiplied $$\frac{1}{12} \times 4 = \frac{4}{12}$$, and since $$\frac{4}{12} = \frac{1}{3}$$, this confirms her division was correct. Choice B incorrectly adds denominators. Choice C repeats the division unnecessarily. Choice D shows the conversion but doesn't complete the verification.
A garden plot covers $$\frac{1}{6}$$ of a yard. If 4 different vegetables are planted in equal sections of this plot, what fraction of the entire yard will the tomato section occupy?
$$\frac{4}{6}$$ of the yard, found by calculating $$\frac{1}{6} \times 4 = \frac{4}{6}$$
$$\frac{1}{2}$$ of the yard, found by calculating $$\frac{1}{6} \div 4 = \frac{4}{6} \div 4 = \frac{1}{6}$$
$$\frac{1}{10}$$ of the yard, found by calculating $$\frac{1}{6} \div 4 = \frac{1}{6+4} = \frac{1}{10}$$
$$\frac{1}{24}$$ of the yard, found by calculating $$\frac{1}{6} \div 4 = \frac{1}{6} \times \frac{1}{4}$$
Explanation
When you see a problem about dividing parts of parts, you need to think step-by-step about what fraction you're working with at each level.
The garden plot covers $$\frac{1}{6}$$ of the yard, and this plot is divided equally among 4 vegetables. To find what fraction of the entire yard each vegetable section occupies, you need to divide the plot's fraction by 4. This means calculating $$\frac{1}{6} \div 4$$.
When dividing by a whole number, you multiply by its reciprocal: $$\frac{1}{6} \div 4 = \frac{1}{6} \times \frac{1}{4} = \frac{1}{24}$$. So the tomato section occupies $$\frac{1}{24}$$ of the entire yard, making D correct.
Let's examine why the other answers are wrong. Choice A incorrectly changes $$\frac{1}{6}$$ to $$\frac{4}{6}$$ before dividing, which doesn't match the problem setup. Choice B multiplies instead of divides ($$\frac{1}{6} \times 4$$), which would give you the area of 4 plots, not one section of a single plot. Choice C uses the wrong division method by adding the numbers in the denominator ($$\frac{1}{6+4}$$), but division of fractions doesn't work this way.
Remember: when you're finding a fraction of a fraction, you multiply the fractions together. Since dividing by 4 is the same as multiplying by $$\frac{1}{4}$$, problems like "What's $$\frac{1}{6}$$ divided by 4?" become multiplication: $$\frac{1}{6} \times \frac{1}{4}$$.
Jake solved $$\frac{1}{7} \div 2$$ and got $$\frac{1}{14}$$. His friend Emma solved $$\frac{1}{14} \div 2$$ and got $$\frac{1}{28}$$. If this pattern continues, what would be the result of $$\frac{1}{28} \div 2$$?
$$\frac{1}{30}$$ because you add 2 to the denominator when dividing by 2
$$\frac{2}{28}$$ because dividing by 2 means multiplying the numerator by 2
$$\frac{1}{56}$$ because the denominator doubles each time when dividing by 2
$$\frac{1}{14}$$ because division and multiplication by 2 are inverse operations
Explanation
When dividing a unit fraction by 2, the denominator doubles: $$\frac{1}{n} \div 2 = \frac{1}{n} \times \frac{1}{2} = \frac{1}{2n}$$. So $$\frac{1}{28} \div 2 = \frac{1}{56}$$. Choice B incorrectly adds 2 to the denominator. Choice C incorrectly multiplies the numerator by 2. Choice D confuses the relationship between operations.
Ben is solving $$\frac{1}{12} \div 3$$ by thinking: "I have $$\frac{1}{12}$$ of a pizza, and I need to split it among 3 people." He draws a circle, shades $$\frac{1}{12}$$ of it, then divides that shaded part into 3 equal sections. What fraction of the original circle is each small section?
$$\frac{1}{9}$$ because $$12 - 3 = 9$$, so each section is $$\frac{1}{9}$$
$$\frac{1}{15}$$ because $$12 + 3 = 15$$, so the answer is $$\frac{1}{15}$$
$$\frac{1}{36}$$ because each section is $$\frac{1}{3}$$ of the $$\frac{1}{12}$$ that was shaded
$$\frac{3}{12}$$ because you multiply the numerator by 3 when dividing by 3
Explanation
When you're dividing fractions, you're finding how many groups of one size fit into another amount, or in this case, how to split a fraction into equal parts.
Ben starts with $$\frac{1}{12}$$ of a pizza and needs to divide it among 3 people. To solve $$\frac{1}{12} \div 3$$, think about what each person gets. Each person receives $$\frac{1}{3}$$ of the original $$\frac{1}{12}$$ piece. To find $$\frac{1}{3}$$ of $$\frac{1}{12}$$, you multiply: $$\frac{1}{3} \times \frac{1}{12} = \frac{1}{36}$$. So each small section represents $$\frac{1}{36}$$ of the original whole circle.
Choice A incorrectly multiplies the numerator by 3, which would make the pieces larger instead of smaller – that doesn't make sense when you're dividing something up. Choice B adds the denominators ($$12 + 3 = 15$$), but addition isn't the right operation here. You're not combining fractions; you're splitting one. Choice D subtracts to get $$\frac{1}{9}$$, but subtraction also doesn't apply to this division situation.
Choice C correctly recognizes that each section is $$\frac{1}{3}$$ of the $$\frac{1}{12}$$ that was shaded, giving $$\frac{1}{36}$$.
Remember: when dividing a fraction by a whole number, you're finding equal parts of that fraction. Multiply the fraction by $$\frac{1}{\text{whole number}}$$ to find each part's size. The result should always be smaller than what you started with when dividing into multiple pieces.
A rope that is $$\frac{1}{10}$$ of a meter long needs to be cut into 8 equal pieces for a craft project. Each piece will be $$\frac{1}{80}$$ meter long. If the craft project actually only needs 6 pieces, how much rope will be left over?
$$\frac{3}{40}$$ meter, because $$\frac{1}{10} \div 6 = \frac{1}{60}$$ and $$\frac{1}{80} \times 6 = \frac{6}{80}$$
$$\frac{1}{40}$$ meter, because $$\frac{1}{80} \times 2 = \frac{2}{80} = \frac{1}{40}$$
$$\frac{1}{20}$$ meter, because $$\frac{1}{10} - \frac{6}{80} = \frac{8}{80} - \frac{6}{80} = \frac{2}{80}$$
$$\frac{2}{80}$$ meter, because 8 pieces minus 6 pieces equals 2 pieces of length $$\frac{1}{80}$$
Explanation
When you encounter word problems involving fractions and leftover amounts, focus on what's actually being used versus what's available.
The rope is $$\frac{1}{10}$$ meter long and gets cut into 8 equal pieces of $$\frac{1}{80}$$ meter each. Since the project only needs 6 pieces, you need to find how much rope those 6 pieces represent, then subtract from the total.
The 6 pieces used equal: $$6 \times \frac{1}{80} = \frac{6}{80}$$ meter
The leftover rope is: $$\frac{1}{10} - \frac{6}{80}$$
To subtract these fractions, convert $$\frac{1}{10}$$ to eighths: $$\frac{1}{10} = \frac{8}{80}$$
So: $$\frac{8}{80} - \frac{6}{80} = \frac{2}{80} = \frac{1}{40}$$ meter
Choice C correctly identifies that 2 unused pieces remain, each $$\frac{1}{80}$$ meter long, giving $$\frac{1}{80} \times 2 = \frac{2}{80} = \frac{1}{40}$$ meter.
Choice A makes the right calculation but stops at $$\frac{2}{80}$$ without simplifying to $$\frac{1}{40}$$. Choice B also gets $$\frac{2}{80}$$ but doesn't reduce the fraction. Choice D uses incorrect logic by dividing the total rope by 6 instead of multiplying the piece length by 6, leading to irrelevant calculations.
Remember: when finding leftover amounts, calculate what's actually used first, then subtract from the total. Always check if your final fraction can be simplified to match the answer choices.
Two students are comparing their solutions to $$\frac{1}{9} \div 7$$. Student A got $$\frac{1}{63}$$ and Student B got $$\frac{7}{9}$$. Which statement best explains the difference in their answers?
Student A correctly divided, while Student B mistakenly multiplied $$\frac{1}{9} \times 7$$ instead
Student B correctly divided, while Student A mistakenly used the reciprocal method incorrectly
Both answers are correct since division problems can have multiple valid solutions depending on the method
Both students made errors; the correct answer should be $$\frac{1}{16}$$ by adding denominators
Explanation
When you encounter a fraction division problem like $$\frac{1}{9} \div 7$$, remember that dividing by a whole number means multiplying by its reciprocal. So $$\frac{1}{9} \div 7$$ becomes $$\frac{1}{9} \times \frac{1}{7} = \frac{1}{63}$$. This shows Student A used the correct method and got the right answer.
Let's trace what likely happened with each student. Student A properly applied the division rule: when dividing a fraction by a whole number, multiply the fraction by the reciprocal of that whole number. Since the reciprocal of 7 is $$\frac{1}{7}$$, multiplying gives $$\frac{1 \times 1}{9 \times 7} = \frac{1}{63}$$.
Student B probably made a common mistake by multiplying instead of dividing. If you calculate $$\frac{1}{9} \times 7$$, you get $$\frac{7}{9}$$, which matches Student B's answer.
Looking at the answer choices: A is wrong because division problems have only one correct answer, not multiple valid solutions. B incorrectly identifies Student A's work as wrong when it's actually correct. C suggests an impossible method of adding denominators (1 + 9 + 7 = 16), which isn't a real division operation. D correctly identifies that Student A divided properly while Student B mistakenly multiplied.
Remember this key principle: $$a \div b = a \times \frac{1}{b}$$. When dividing by any number, you multiply by its reciprocal instead. This rule will help you avoid the common trap of accidentally multiplying when you should be dividing.
Mrs. Chen has $$\frac{1}{8}$$ pound of clay. She wants to make 5 identical sculptures. After dividing the clay equally, she realizes each sculpture will use $$\frac{1}{40}$$ pound of clay. How can she check if this amount is reasonable?
Compare $$\frac{1}{40}$$ to $$\frac{1}{8}$$ and verify that $$\frac{1}{40}$$ is smaller, which makes sense
Divide $$\frac{1}{40}$$ by 5 again to get $$\frac{1}{200}$$ and see if this seems like enough clay
Multiply $$\frac{1}{40} \times 5 = \frac{5}{40} = \frac{1}{8}$$ to confirm it equals her starting amount
Add $$\frac{1}{40} + 5 = \frac{1}{40} + \frac{200}{40} = \frac{201}{40}$$ and check if this is reasonable
Explanation
To verify division, multiply the quotient by the divisor to see if you get the dividend: $$\frac{1}{40} \times 5 = \frac{5}{40} = \frac{1}{8}$$, which confirms the division is correct. Choice A only checks reasonableness but not accuracy. Choice C incorrectly adds instead of multiplying. Choice D performs unnecessary additional division.
A baker has $\tfrac{1}{4}$ of a cake left. She shares it equally in two different ways: (1) among 2 kids and (2) among 4 kids. Imagine the cake is first divided into 4 equal pieces, and then that one piece is partitioned again into equal smaller parts. Dividing a fraction by a whole number creates smaller equal parts. Which statement is correct when you compare $\tfrac{1}{4} \div 2$ and $\tfrac{1}{4} \div 4$?
$\tfrac{1}{4} \div 2$ and $\tfrac{1}{4} \div 4$ are equal because the starting fraction is the same.
$\tfrac{1}{4} \div 4$ is larger than $\tfrac{1}{4} \div 2$ because more kids means more cake per kid.
$\tfrac{1}{4} \div 4$ is smaller than $\tfrac{1}{4} \div 2$ because the same $\tfrac{1}{4}$ is split into more equal parts.
$\tfrac{1}{4} \div 4$ is larger than $\tfrac{1}{4}$ because division makes numbers bigger.
Explanation
Unit fractions, which have a numerator of 1, can be divided by whole numbers to find smaller equal shares. Sharing ($\frac{1}{4}$) of a cake among 2 kids gives ($\frac{1}{8}$) each, while among 4 kids gives ($\frac{1}{16}$) each. This partitioning shows the ($\frac{1}{4}$) divided into more parts for more kids, making shares smaller. Visualizing the cake quartered, then one quarter subdivided into 2 or 4 pieces, highlights the size difference. A misconception is that more kids mean larger shares, but actually, it means smaller ones. Generally, larger divisors make the resulting fraction smaller. This illustrates how division scales down unit fractions proportionally.
A student says: “To find $\tfrac{1}{4} \div 2$, you just divide the denominator by 2, so the answer is $\tfrac{1}{2}$.” The situation is splitting $\tfrac{1}{4}$ of a pizza equally between 2 people, and dividing a fraction by a whole number creates smaller equal parts because you partition the $\tfrac{1}{4}$ again. Which claim about the result is incorrect?
The result must be smaller than $\tfrac{1}{4}$ because the $\tfrac{1}{4}$ is being split.
The answer is $\tfrac{1}{2}$ of the whole pizza.
Each person gets $\tfrac{1}{8}$ of the whole pizza.
The $\tfrac{1}{4}$ piece is partitioned into 2 equal parts.
Explanation
Dividing a unit fraction by a whole number means splitting that fraction into even smaller equal parts. In the context of sharing, if you have one-fourth of a pizza and split it equally between 2 people, each gets an equal share of that fourth. This partitioning further divides the fourth into 2 equal pieces, resulting in each being one-eighth of the pizza. Visually, picture a pizza quartered, then taking one quarter and halving it, so each half is 1/8 of the whole. A common misconception is thinking you divide the denominator to get a larger fraction like one-half, but that's incorrect. In general, proper division multiplies the denominator instead. This ensures the result is a smaller fraction than the original.
A sandwich is cut so that you have $\tfrac{1}{2}$ of a sandwich left. You want to share that $\tfrac{1}{2}$ equally among 3 students. Imagine the half-sandwich is first shown as 1 of 2 equal parts of the whole, and then that half is partitioned into 3 equal smaller parts. Dividing a fraction by a whole number creates smaller equal parts. What is the result of $\tfrac{1}{2} \div 3$?
$\tfrac{1}{1}$ of a sandwich
$\tfrac{1}{2}$ of a sandwich
$\tfrac{3}{2}$ of a sandwich
$\tfrac{1}{6}$ of a sandwich
Explanation
Unit fractions, which have a numerator of 1, can be divided by whole numbers to create smaller equal shares. In this scenario, dividing 1/2 of a sandwich by 3 means sharing that half equally among 3 students. This involves taking the 1/2 and partitioning it further into 3 equal smaller parts. Visually, you can draw a whole sandwich divided into 2 halves, then divide one half into 3 equal sections, each representing 1/6 of the whole. A common misconception is that dividing by 3 would make each share larger than 1/2, but it actually makes them smaller. In general, dividing a unit fraction by a whole number results in a smaller fraction, with the denominator becoming the product of the original denominator and the divisor. Therefore, 1/2 ÷ 3 equals 1/6 of a sandwich, which is choice A.