PSLE
At first, the ratio of the number of boys to the number of girls in a kindergarten was 1 : 3. After 21 girls and 21 boys joined the kindergarten, the ratio became 2 : 3.

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

	Boys	Girls	Difference	Total
Before	1x1 = 1 u	3x1  = 3 u	2x1 = 2 u	1 u + 3 u = 4 u
Change	+ 21	+ 21
After	2x2 = 4 u	3x2 = 6 u	1x2 = 2 u	4 u + 6 u = 10 u

(a)
21 girls and 21 boys joined the kindergarten. There was equal number of girls and boys who joined the kindergarten.

Since the increase in number of students 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 1 = 2 u

Percent of students who were girls at first
= ³₄ x 100%
≈ 75% (Round off to whole number.)

Percent of students who are boys in the end
= ⁶₁₀ x 100%
≈ 60% (Round off to whole number.)

Percent of students who were girls decreased from 75% to 60%. (2)

(b)
Increase in the number of girls
= 6 u - 3 u
= 3 u

3 u = 21

1 u = 21 ÷ 3 = 7

Number of students in the end
= 10 u
= 10 x 7
= 70

Answer(s): (a) 2; (b) 70