Week 24 · Estimating, fair shares and area · 3 lessons
2
Thirds, fourths and liters
Discover Mathematics P3: Term 2, Lessons 91–100
GoalCompare thirds and fourths of the same whole and write equal amounts of a litre as different fractions.
New words
third · one of 3 equal parts, 1/3fourth · one of 4 equal parts, 1/4litre (L) · a unit for liquids; 1 L = 1,000 mL
Explanation
Imagine two identical flatbreads. Omar cuts his into 3 equal parts and Mariam cuts hers into 4 equal parts. Each takes one part. The more parts we cut, the smaller each part is. So a third is bigger than a fourth: 1/3 > 1/4. If 3 friends share a loaf fairly each gets 1/3, and if 4 friends share the same size loaf each gets 1/4. Fractions also work for liquids. One litre is 1,000 mL. Half a litre is 1,000 ÷ 2 = 500 mL. A fourth of a litre is 1,000 ÷ 4 = 250 mL. Two fourths is 2 × 250 = 500 mL, which is the same amount as 1/2 L. So 1/2 L = 2/4 L = 500 mL. An eighth of a litre is 1,000 ÷ 8 = 125 mL, and 4/8 L is also 500 mL.
Worked examples
Omar cuts one flatbread into thirds and Mariam cuts an identical one into fourths. Each takes one piece. Omar’s piece is bigger, because the loaf was cut into fewer parts: 1/3 > 1/4.
A bottle holds 1 L, which is 1,000 mL. Pour out half of it. Each half is 1/2 L, which is 500 mL.
Fraction of a litre
Work
Amount
1/2 L
1,000 ÷ 2
500 mL
1/4 L
1,000 ÷ 4
250 mL
2/4 L
2 × 250
500 mL
1/8 L
1,000 ÷ 8
125 mL
4/8 L
4 × 125
500 mL
Different fractions of a litre can be the same amount. Work out each one with division and multiplication. 1/2 L, 2/4 L and 4/8 L are all 500 mL.
Class activity
Juice jug. Find a bottle that holds 1 litre and a cup that holds 250 mL (look at the label). Fill the cup from the bottle again and again. How many cups empty the bottle? Then pour out 2 cups and compare what is left with half of the bottle. Write what you find with fractions.
Practice
1
Four friends share one loaf fairly. What fraction of the loaf does each friend get?
Each friend gets 1/4 of the loaf.
2
Which is bigger, 1/3 of a loaf or 1/4 of an identical loaf? Why?
1/3 is bigger. A loaf cut into 3 parts has bigger parts than a loaf cut into 4 parts.
3
How many millilitres are in 1/2 litre?
1,000 ÷ 2 = 500 mL.
4
A 1-litre jug is divided into 4 equal parts, as shown. Each part is 1/4 L. How many mL is each part?
1,000 ÷ 4 = 250 mL.
5
Seif drinks 2/4 L of juice and Hana drinks 1/2 L of the same juice. Who drinks more?
Nobody. 2/4 L = 500 mL and 1/2 L = 500 mL, so they drink the same amount.
