PSLE
At first, the ratio of the number of boys to the number of girls in a school was 4 : 3. After 36 boys and 36 girls left the school, the ratio became 10 : 3.

1. Did the percentage of pupils 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 pupils were there in the school?

	Girls	Boys	Difference	Total
Before	3x7 = 21 u	4x7  = 28 u	1x7 = 7 u	21 u + 28 u = 49 u
Change	- 36	- 36
After	3x1 = 3 u	10x1 = 10 u	7x1 = 7 u	3 u + 10 u = 13 u

(a)
36 boys and 36 girls left the school. There was equal number of boys and girls who left the school.

Since the decrease in number of pupils 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 1 and 7 = 7 u

Percent of pupils who were boys at first
= ²⁸₄₉ x 100%
≈ 57% (Round off to whole number.)

Percent of pupils who are boys in the end
= ¹⁰₁₃ x 100%
≈ 77% (Round off to whole number.)

Percent of pupils who were boys increased from 57% to 77%. (1)

(b)
Decrease in the number of boys
= 10 u - 28 u
= 18 u

18 u = 36

1 u = 36 ÷ 18 = 2

Number of pupils at first
= 49 u
= 49 x 2
= 98

Answer(s): (a) 1; (b) 98