PSLE
At first, the ratio of the number of girls to the number of boys in a school was 3 : 2. After 21 girls and 21 boys joined the school, the ratio became 10 : 9.

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 school?

	Girls	Boys	Difference	Total
Before	3x1  = 3 u	2x1 = 2 u	1x1 = 1 u	3 u + 2 u = 5 u
Change	+ 21	+ 21
After	10x1 = 10 u	9x1 = 9 u	1x1 = 1 u	10 u + 9 u = 19 u

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

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 1 and 1 = 1 u

Percent of students who were boys at first
= ²₅ x 100%
≈ 40% (Round off to whole number.)

Percent of students who are boys in the end
= ⁹₁₉ x 100%
≈ 47% (Round off to whole number.)

Percent of students who were boys increased from 40% to 47%. (1)

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

7 u = 21

1 u = 21 ÷ 7 = 3

Number of girls in the end
= 10 u
= 10 x 3
= 30

Answer(s): (a) 1; (b) 30