PSLE
At first, the ratio of the number of girls to the number of boys in a kindergarten was 3 : 1. After 15 girls and 15 boys left the kindergarten, 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 kindergarten?

	Boys	Girls	Difference	Total
Before	1x7 = 7 u	3x7  = 21 u	2x7 = 14 u	7 u + 21 u = 28 u
Change	- 15	- 15
After	2x2 = 4 u	9x2 = 18 u	7x2 = 14 u	4 u + 18 u = 22 u

(a)
15 girls and 15 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 2 and 7 = 14 u

Percent of pupils who were girls at first
= ²¹₂₈ x 100%
≈ 75% (Round off to whole number.)

Percent of pupils who are girls in the end
= ¹⁸₂₂ x 100%
≈ 82% (Round off to whole number.)

Percent of pupils who were girls increased from 75% to 82%. (1)

(b)
Decrease in the number of girls
= 18 u - 21 u
= 3 u

3 u = 15

1 u = 15 ÷ 3 = 5

Number of pupils at first
= 28 u
= 28 x 5
= 140

Answer(s): (a) 1; (b) 140