Multiplying fractions

Advanced Math 5 · Unit 8 · fraction-mul-div · Teacher edition

fraction-mul-div

Name ______________________________Date ______________

1.
Compute $\dfrac{3}{4} \times \dfrac{2}{5}$ .
Answer: ______________
2.
A ribbon $4$ m long is cut into pieces each $\dfrac{1}{5}$ m long. How many pieces are there?
Answer: ______________
3.
A garden plot is $\dfrac{2}{3}$ m wide and $\dfrac{4}{5}$ m long. What is its area, in m²?
1. (A)
  $\frac{3}{4}$
2. (B)
  $\frac{22}{15}$
3. (C)
  $\frac{8}{15}$
4. (D)
  $\frac{2}{5}$

Answer key — Multiplying fractions

1.
3/10fraction-mul-div-01
$\frac{3}{4} \times \frac{2}{5} = \frac{6}{20} = \mathbf{\frac{3}{10}}$ . Cross-cancelling first: $\frac{3}{2} \times \frac{1}{5} = \frac{3}{10}$ .
2.
20fraction-mul-div-02
$4 \div \frac{1}{5} = 4 \times 5 = \mathbf{20}$ pieces. Check: $20 \times \frac{1}{5} = 4$ . ✓ (Dividing by a unit fraction makes the answer bigger; $\frac{4}{5}$ would be multiplying.)
3.
(C)
$\frac{8}{15}$
fraction-mul-div-03
$\frac{2}{3} \times \frac{4}{5} = \frac{2 \times 4}{3 \times 5} = \mathbf{\frac{8}{15}}$ m². ( $\frac{3}{4} = \frac{6}{8}$ comes from adding the tops and adding the bottoms; $\frac{22}{15}$ is the sum $\frac{2}{3} + \frac{4}{5}$ ; $\frac{2}{5}$ is a cancelling slip.)