PSLE
At first, the ratio of the number of boys to the number of girls in a school was 8 : 3. After 40 boys and 40 girls left the school, the ratio became 4 : 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. At first, how many students were there in the school?

	Girls	Boys	Difference	Total
Before	3x3 = 9 u	8x3  = 24 u	5x3 = 15 u	9 u + 24 u = 33 u
Change	- 40	- 40
After	1x5 = 5 u	4x5 = 20 u	3x5 = 15 u	5 u + 20 u = 25 u

(a)
40 boys and 40 girls left the school. There was equal number of boys and girls who left the school.

Since the decrease 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 5 and 3 = 15 u

Percent of students who were boys at first
= ²⁴₃₃ x 100%
≈ 73% (Round off to whole number.)

Percent of students who are boys in the end
= ²⁰₂₅ x 100%
≈ 80% (Round off to whole number.)

Percent of students who were boys increased from 73% to 80%. (1)

(b)
Decrease in the number of boys
= 20 u - 24 u
= 4 u

4 u = 40

1 u = 40 ÷ 4 = 10

Number of students at first
= 33 u
= 33 x 10
= 330

Answer(s): (a) 1; (b) 330