Fractions with the Same Denominator
Primary 4 · Mathematics
Fractions with the Same Denominator
Lesson 4 of 5
30 min
Let's begin
Every morning Byiringiro drinks 18 of a litre of milk. How much milk does he drink in 3 days? And his little sister drinks half of what is in a cup that is 13 full. How much is that? Both questions are about multiplying fractions.
Multiplication is repeated addition, so 3 × 18 means 18 + 18 + 18 = 38. Byiringiro drinks 38 of a litre in 3 days.
Shortcut: multiply the numerator by the whole number and keep the denominator.
The word "of" means multiply. To find 34 of 12 mangoes, first find 14 of 12 by dividing: 12 ÷ 4 = 3. Then take 3 of those quarters: 3 × 3 = 9 mangoes.
Divide by the bottom, multiply by the top.
12 × 13 means "one half of one third". Draw a rectangle, cut it into 3 equal columns and shade one column (that is 13). Now cut that column in half:
The pink piece is one of 6 equal pieces of the whole rectangle, so 12 × 13 = 16. Byiringiro's sister drinks 16 of a full cup.
Shortcut: multiply the numerators, and multiply the denominators.
Tip
Top times top, bottom times bottom. For a fraction times a fraction, multiply straight across. For a whole number times a fraction, multiply only the top.
Work each one in your exercise book first. Then tap Show answer to check.
4 × 19
Answer: 49
3 × 27
Answer: 67
What is 23 of 15?
Answer: 10. 15 ÷ 3 = 5, and 5 × 2 = 10.
12 × 15
Answer: 110
23 × 45
Answer: 815
What you learned today
Sources and credits
The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.
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