PSLE
At first, the ratio of the number of girls to the number of boys in a school was 2 : 1. After 20 girls and 20 boys left the school, the ratio became 9 : 2.

1. Did the percentage of pupils who were girls increase (1), decrease (2) or remain the same (3)? Give your answer in number. (Eg Increase = 1)
2. At first, how many pupils were there in the school?

	Boys	Girls	Difference	Total
Before	1x7 = 7 u	2x7  = 14 u	1x7 = 7 u	7 u + 14 u = 21 u
Change	- 20	- 20
After	2x1 = 2 u	9x1 = 9 u	7x1 = 7 u	2 u + 9 u = 11 u

(a)
20 girls and 20 boys left the school. There was equal number of girls and boys who left the school.

Since the decrease in number of pupils in both groups is the same, the difference between the number of girls and boys remains unchanged.

We make the difference between the number of girls and boys at first and in the end the same.
LCM of 1 and 7 = 7 u

Percent of pupils who were girls at first
= ¹⁴₂₁ x 100%
≈ 67% (Round off to whole number.)

Percent of pupils who are girls in the end
= ⁹₁₁ x 100%
≈ 82% (Round off to whole number.)

Percent of pupils who were girls increased from 67% to 82%. (1)

(b)
Decrease in the number of girls
= 9 u - 14 u
= 5 u

5 u = 20

1 u = 20 ÷ 5 = 4

Number of pupils at first
= 21 u
= 21 x 4
= 84

Answer(s): (a) 1; (b) 84