PSLE
At first, the ratio of the number of girls to the number of boys in a kindergarten was 5 : 9. After 12 boys and 12 girls joined the kindergarten, the ratio became 2 : 3.

1. Did the percentage of students 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 students were there in the kindergarten?

	Girls	Boys	Difference	Total
Before	5x1 = 5 u	9x1  = 9 u	4x1 = 4 u	5 u + 9 u = 14 u
Change	+ 12	+ 12
After	2x4 = 8 u	3x4 = 12 u	1x4 = 4 u	8 u + 12 u = 20 u

(a)
12 boys and 12 girls joined the kindergarten. There was equal number of boys and girls who joined the kindergarten.

Since the increase in number of students 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 4 and 1 = 4 u

Percent of students who were boys at first
= ⁹₁₄ x 100%
≈ 64% (Round off to whole number.)

Percent of students who are girls in the end
= ¹²₂₀ x 100%
≈ 60% (Round off to whole number.)

Percent of students who were boys decreased from 64% to 60%. (2)

(b)
Increase in the number of boys
= 12 u - 9 u
= 3 u

3 u = 12

1 u = 12 ÷ 3 = 4

Number of students in the end
= 20 u
= 20 x 4
= 80

Answer(s): (a) 2; (b) 80