PSLE
At first, the ratio of the number of girls to the number of boys in a kindergarten was 7 : 2. After 18 girls and 18 boys joined the kindergarten, the ratio became 2 : 1.

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 girls were there in the kindergarten?

	Girls	Boys	Difference	Total
Before	7x1  = 7 u	2x1 = 2 u	5x1 = 5 u	7 u + 2 u = 9 u
Change	+ 18	+ 18
After	2x5 = 10 u	1x5 = 5 u	1x5 = 5 u	10 u + 5 u = 15 u

(a)
18 girls and 18 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 5 and 1 = 5 u

Percent of students who were boys at first
= ²₉ x 100%
≈ 22% (Round off to whole number.)

Percent of students who are boys in the end
= ⁵₁₅ x 100%
≈ 33% (Round off to whole number.)

Percent of students who were boys increased from 22% to 33%. (1)

(b)
Increase in the number of girls
= 10 u - 7 u
= 3 u

3 u = 18

1 u = 18 ÷ 3 = 6

Number of girls in the end
= 10 u
= 10 x 6
= 60

Answer(s): (a) 1; (b) 60