PSLE
At first, the ratio of the number of boys to the number of girls in a school was 2 : 1. After 30 boys and 30 girls joined the school, the ratio became 9 : 7.

1. Did the percentage of students 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	2x2  = 4 u	1x2 = 2 u	1x2 = 2 u	4 u + 2 u = 6 u
Change	+ 30	+ 30
After	9x1 = 9 u	7x1 = 7 u	2x1 = 2 u	9 u + 7 u = 16 u

(a)
30 boys and 30 girls joined the school. There was equal number of boys and girls who joined the school.

Since the increase 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 1 and 2 = 2 u

Percent of students who were girls at first
= ²₆ x 100%
≈ 33% (Round off to whole number.)

Percent of students who are girls in the end
= ⁷₁₆ x 100%
≈ 44% (Round off to whole number.)

Percent of students who were girls increased from 33% to 44%. (1)

(b)
Increase in the number of boys
= 9 u - 4 u
= 5 u

5 u = 30

1 u = 30 ÷ 5 = 6

Number of boys in the end
= 9 u
= 9 x 6
= 54

Answer(s): (a) 1; (b) 54