Percentages and Ratios
Primary 6 · Mathematics
Percentages and Ratios
Lesson 2 of 6
15 min
Let's begin
On the last Saturday of the month, 240 people came to umuganda in Mugisha's umudugudu. The village leader says 35% of them were children. How many children cleaned the roads that morning?
To answer, you need to find a percentage of a quantity. You will also learn the opposite skill: saying what percentage one amount is of another.
"Of" means multiply. Change the percentage into a fraction or a decimal, then multiply.
Worked example
Find 35% of 240 people.
35% of 240 = 35100 × 240 = 8 400 ÷ 100 = 84.
So 84 children came to umuganda.
Check it another way, with benchmark percentages.
10% of 240 = 240 ÷ 10 = 24
30% = 3 × 24 = 72
5% = half of 10% = 12
35% = 30% + 5% = 72 + 12 = 84. The same answer.
Worked example
A pair of shoes costs 6 000 Frw. What is 15% of the price?
10% of 6 000 = 600 Frw. 5% = 300 Frw. So 15% = 600 + 300 = 900 Frw.
Make a fraction: the part over the whole. Then multiply by 100.
Worked example
Ishimwe scored 36 out of 40. What percentage is that?
3640 × 100 = 3 600 ÷ 40 = 90. Ishimwe scored 90%.
Worked example
A class has 45 pupils and 18 of them are boys. What percentage are boys?
1845 × 100 = 1 800 ÷ 45 = 40. So 40% are boys, and 100% − 40% = 60% are girls.
Watch the units. What percentage of 1.5 kg is 300 g? First make the units the same: 1.5 kg = 1 500 g. Then 300 ÷ 1 500 × 100 = 20%.
At School A, 72 of the 80 P6 pupils passed a test. At School B, 115 of the 125 P6 pupils passed. Which school did better?
School A: 72 ÷ 80 × 100 = 90%.
School B: 115 ÷ 125 × 100 = 92%.
School B did better. The fair reason is its higher percentage, not that "more pupils passed".
Uwase has spent 60% of her pocket money, and that was 1 800 Frw. How much did she have at first?
60% is 1 800 Frw, so 1% is 1 800 ÷ 60 = 30 Frw.
100% is 30 × 100 = 3 000 Frw.
Tip
Stuck? Find 10% first by dividing by 10. You can build almost any percentage from 10%, 5% and 1%.
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