PSLE
At first, the ratio of the number of boys to the number of girls in a kindergarten was 5 : 2. After 10 boys and 10 girls left the kindergarten, the ratio became 5 : 1.

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. In the end, how many girls were there in the kindergarten?

	Boys	Girls	Difference	Total
Before	5x4  = 20 u	2x4 = 8 u	3x4 = 12 u	20 u + 8 u = 28 u
Change	- 10	- 10
After	5x3 = 15 u	1x3 = 3 u	4x3 = 12 u	15 u + 3 u = 18 u

(a)
10 boys and 10 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 3 and 4 = 12 u

Percent of pupils who were girls at first
= ⁸₂₈ x 100%
≈ 29% (Round off to whole number.)

Percent of pupils who are girls in the end
= ³₁₈ x 100%
≈ 17% (Round off to whole number.)

Percent of pupils who were girls decreased from 29% to 17%. (2)

(b)
Decrease in the number of boys
= 15 u - 20 u
= 5 u

5 u = 10

1 u = 10 ÷ 5 = 2

Number of girls in the end
= 3 u
= 3 x 2
= 6

Answer(s): (a) 2; (b) 6