PSLE
At first, the ratio of the number of girls to the number of boys in a kindergarten was 7 : 4. After 40 girls and 40 boys left the kindergarten, the ratio became 3 : 1.

1. Did the percentage of pupils who were boys increase (1), decrease (2) or remain the same (3)? Give your answer in number. (Eg Increase = 1)
2. In the end, how many boys were there in the kindergarten?

	Girls	Boys	Difference	Total
Before	7x2  = 14 u	4x2 = 8 u	3x2 = 6 u	14 u + 8 u = 22 u
Change	- 40	- 40
After	3x3 = 9 u	1x3 = 3 u	2x3 = 6 u	9 u + 3 u = 12 u

(a)
40 girls and 40 boys left the kindergarten. There was equal number of girls and boys who left the kindergarten.

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 3 and 2 = 6 u

Percent of pupils who were boys at first
= ⁸₂₂ x 100%
≈ 36% (Round off to whole number.)

Percent of pupils who are boys in the end
= ³₁₂ x 100%
≈ 25% (Round off to whole number.)

Percent of pupils who were boys decreased from 36% to 25%. (2)

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

5 u = 40

1 u = 40 ÷ 5 = 8

Number of boys in the end
= 3 u
= 3 x 8
= 24

Answer(s): (a) 2; (b) 24