PSLE
At first, the ratio of the number of girls to the number of boys in a school was 7 : 6. After 20 girls and 20 boys left the school, the ratio became 5 : 4.

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 boys were there in the school?

	Girls	Boys	Difference	Total
Before	7x1  = 7 u	6x1 = 6 u	1x1 = 1 u	7 u + 6 u = 13 u
Change	- 20	- 20
After	5x1 = 5 u	4x1 = 4 u	1x1 = 1 u	5 u + 4 u = 9 u

(a)
20 girls and 20 boys left the school. There was equal number of girls and boys who left the school.

Since the decrease 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
= ⁶₁₃ x 100%
≈ 46% (Round off to whole number.)

Percent of students who are boys in the end
= ⁴₉ x 100%
≈ 44% (Round off to whole number.)

Percent of students who were boys decreased from 46% to 44%. (2)

(b)
Decrease in the number of girls
= 5 u - 7 u
= 2 u

2 u = 20

1 u = 20 ÷ 2 = 10

Number of boys in the end
= 4 u
= 4 x 10
= 40

Answer(s): (a) 2; (b) 40