PSLE
At first, the ratio of the number of girls to the number of boys in a school was 2 : 5. After 28 boys and 28 girls joined the school, the ratio became 2 : 3.

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 pupils were there in the school?

	Girls	Boys	Difference	Total
Before	2x1 = 2 u	5x1  = 5 u	3x1 = 3 u	2 u + 5 u = 7 u
Change	+ 28	+ 28
After	2x3 = 6 u	3x3 = 9 u	1x3 = 3 u	6 u + 9 u = 15 u

(a)
28 boys and 28 girls joined the school. There was equal number of boys and girls who joined the school.

Since the increase 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 1 = 3 u

Percent of pupils who were boys at first
= ⁵₇ x 100%
≈ 71% (Round off to whole number.)

Percent of pupils who are girls in the end
= ⁹₁₅ x 100%
≈ 60% (Round off to whole number.)

Percent of pupils who were boys decreased from 71% to 60%. (2)

(b)
Increase in the number of boys
= 9 u - 5 u
= 4 u

4 u = 28

1 u = 28 ÷ 4 = 7

Number of pupils in the end
= 15 u
= 15 x 7
= 105

Answer(s): (a) 2; (b) 105