PSLE
At first, the ratio of the number of boys to the number of girls in a school was 4 : 1. After 8 boys and 8 girls joined the school, the ratio became 10 : 3.

1. Did the percentage of pupils who were girls 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?

	Boys	Girls	Difference	Total
Before	4x7  = 28 u	1x7 = 7 u	3x7 = 21 u	28 u + 7 u = 35 u
Change	+ 8	+ 8
After	10x3 = 30 u	3x3 = 9 u	7x3 = 21 u	30 u + 9 u = 39 u

(a)
8 boys and 8 girls joined the school. There was equal number of boys and girls who joined the school.

Since the increase 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 3 and 7 = 21 u

Percent of pupils who were girls at first
= ⁷₃₅ x 100%
≈ 20% (Round off to whole number.)

Percent of pupils who are girls in the end
= ⁹₃₉ x 100%
≈ 23% (Round off to whole number.)

Percent of pupils who were girls increased from 20% to 23%. (1)

(b)
Increase in the number of boys
= 30 u - 28 u
= 2 u

2 u = 8

1 u = 8 ÷ 2 = 4

Number of boys in the end
= 30 u
= 30 x 4
= 120

Answer(s): (a) 1; (b) 120