PSLE
At first, the ratio of the number of boys to the number of girls in a kindergarten was 7 : 5. After 15 boys and 15 girls left the kindergarten, the ratio became 2 : 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. At first, how many pupils were there in the kindergarten?

	Girls	Boys	Difference	Total
Before	5x1 = 5 u	7x1  = 7 u	2x1 = 2 u	5 u + 7 u = 12 u
Change	- 15	- 15
After	1x2 = 2 u	2x2 = 4 u	1x2 = 2 u	2 u + 4 u = 6 u

(a)
15 boys and 15 girls left the kindergarten. There was equal number of boys and girls who left the kindergarten.

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

We make the difference between the number of boys and girls at first and in the end the same.
LCM of 2 and 1 = 2 u

Percent of pupils who were boys at first
= ⁷₁₂ x 100%
≈ 58% (Round off to whole number.)

Percent of pupils who are boys in the end
= ⁴₆ x 100%
≈ 67% (Round off to whole number.)

Percent of pupils who were boys increased from 58% to 67%. (1)

(b)
Decrease in the number of boys
= 4 u - 7 u
= 3 u

3 u = 15

1 u = 15 ÷ 3 = 5

Number of pupils at first
= 12 u
= 12 x 5
= 60

Answer(s): (a) 1; (b) 60