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Area model fraction × fraction

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5100455
Sarah has a supply of sugar. She uses \(\frac{4}{9}\) of the original amount, then later uses \(\frac{7}{10}\) of what remained. What fraction of the original amount is left? Explain how an area model represents the fraction-of-a-fraction multiplication.

Hints

- First find the fraction of the original amount left after the first use. - What fraction of that remainder is left after \(\frac{7}{10}\) is used? - In an area model, the overlap represents a fraction of a fraction.

Solution

1. After the first use, \(1-\frac{4}{9}=\frac{5}{9}\) of the original sugar remains. 2. After the second use, \(\frac{3}{10}\) of that remaining amount is left. 3. In an area model, one dimension represents \(\frac{5}{9}\) of the original amount and the other represents keeping \(\frac{3}{10}\) of that part. The overlap represents \(\frac{5}{9}\times\frac{3}{10}=\frac{15}{90}=\frac{1}{6}\) of the original amount.

Answer

\(\frac{1}{6}\) of the original amount is left. In an area model, the overlap of \(\frac{5}{9}\) in one direction and \(\frac{3}{10}\) in the other represents this fraction of the original amount.
5107725
Calculate each fraction of a quantity. Explain how an area model represents each multiplication. For parts a) and b), also give a short real-life example that could match the expression. a) \(\frac{4}{5}\) of \(\frac{1}{2}\,\text{gal}\) b) \(\frac{2}{3}\) of \(1 \frac{1}{4}\,\text{lb}\) c) \(\frac{3}{10}\) of \(\frac{5}{6}\,\text{yd}\)

Hints

- The word “of” indicates multiplication in these fraction-of-a-quantity problems. - Multiply the numerators and multiply the denominators when multiplying two fractions. - Rewrite the mixed number as an improper fraction before multiplying. - In an area model, the overlap represents the product of the two fractions. - For the real-life examples, think of using or taking only part of an amount that is already a fraction of a whole.

Solution

1. For a), the area-model overlap represents \(\frac{4}{5}\times\frac{1}{2}=\frac{2}{5}\), so the amount is \(\frac{2}{5}\,\text{gal}\). One example is drinking \(\frac{4}{5}\) of the juice in a half-gallon container. 2. For b), rewrite \(1 \frac{1}{4}\) as \(\frac{5}{4}\). The overlap represents \(\frac{2}{3}\times\frac{5}{4}=\frac{5}{6}\), so the amount is \(\frac{5}{6}\,\text{lb}\). One example is using \(\frac{2}{3}\) of a \(1 \frac{1}{4}\)-pound bag of flour. 3. For c), the overlap represents \(\frac{3}{10}\times\frac{5}{6}=\frac{1}{4}\), so the length is \(\frac{1}{4}\,\text{yd}\).

Answer

a) \(\frac{2}{5}\,\text{gal}\). The area-model overlap represents \(\frac{4}{5}\times\frac{1}{2}\). One example is drinking \(\frac{4}{5}\) of the juice in a half-gallon container. b) \(\frac{5}{6}\,\text{lb}\). The overlap represents \(\frac{2}{3}\times\frac{5}{4}\). One example is using \(\frac{2}{3}\) of a \(1 \frac{1}{4}\)-pound bag of flour. c) \(\frac{1}{4}\,\text{yd}\). The overlap represents \(\frac{3}{10}\times\frac{5}{6}\).
5107735
Compare the two quantities. Explain how an area model represents each fraction-of-a-fraction product, then decide whether one quantity is greater or whether they are equal. Quantity A: \(\frac{3}{4}\) of \(\frac{2}{5}\,\text{lb}\) Quantity B: \(\frac{1}{2}\) of \(\frac{3}{5}\,\text{lb}\)

Hints

- Write each “fraction of a fraction” as multiplication. - Use the overlap in each area model to represent the product. - Simplify both products before comparing them.

Solution

1. For Quantity A, the area-model overlap represents \(\frac{3}{4}\times\frac{2}{5}=\frac{6}{20}=\frac{3}{10}\), so A is \(\frac{3}{10}\,\text{lb}\). 2. For Quantity B, the overlap represents \(\frac{1}{2}\times\frac{3}{5}=\frac{3}{10}\), so B is \(\frac{3}{10}\,\text{lb}\). 3. The quantities are equal.

Answer

The quantities are equal; each is \(\frac{3}{10}\,\text{lb}\). In the two area models, the overlaps represent \(\frac{3}{4}\times\frac{2}{5}\) and \(\frac{1}{2}\times\frac{3}{5}\), and both products equal \(\frac{3}{10}\).
5107765
At a school field day, \(60\) students participate. Two fifths choose track and field. One half of those students compete in the long jump. a) Explain how an area model shows what fraction of all the students are long jumpers. b) Show two different ways to find how many students compete in the long jump.

Hints

- The long jumpers are a fraction of the track-and-field group, which is itself a fraction of all students. - Use the area-model overlap for part a). - For part b), compare finding the overall fraction first with finding the groups step by step.

Solution

1. For a), the area-model overlap represents \(\frac{2}{5}\times\frac{1}{2}=\frac{1}{5}\) of all the students. 2. One method for b) is to use the overall fraction: \(\frac{1}{5}\) of \(60\) is \(12\). 3. Another method is to find \(\frac{2}{5}\) of \(60\), which is \(24\), and then take half of \(24\), which is \(12\).

Answer

a) \(\frac{1}{5}\). In an area model, the overlap of \(\frac{2}{5}\) in one direction and \(\frac{1}{2}\) in the other represents \(\frac{1}{5}\) of all students. b) \(12\) students. One method is \(\frac{1}{5}\) of \(60\). Another is to find \(\frac{2}{5}\) of \(60\), which is \(24\), and then take \(\frac{1}{2}\) of \(24\).
5358215
A farmer fences off part of a rectangular pasture for sheep. The sheep area is \(\frac{4}{5}\) of the pasture's width and \(\frac{2}{3}\) of its height, as shown in the grid. What fraction of the pasture's total area is used by the sheep?
Figure for problem 535821

Hints

- The area fraction comes from multiplying the fraction of the width by the fraction of the height. - Use the rectangular grid as an area model for the two fractions. - Count the total cells and the cells in the overlap.

Solution

1. The sheep area uses \(\frac{4}{5}\) of the width and \(\frac{2}{3}\) of the height. 2. Use the area model to multiply the fractions: \(\frac{4}{5}\times\frac{2}{3}=\frac{8}{15}\). 3. The grid has \(15\) equal cells, and the overlap covers \(8\) of them, confirming \(\frac{8}{15}\).

Answer

\(\frac{8}{15}\) of the pasture's total area

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