PSLE
At first, the ratio of the number of girls to the number of boys in a school was 10 : 3. After 14 girls and 14 boys left the school, the ratio became 4 : 1.

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. At first, how many students were there in the school?

	Boys	Girls	Difference	Total
Before	3x3 = 9 u	10x3  = 30 u	7x3 = 21 u	9 u + 30 u = 39 u
Change	- 14	- 14
After	1x7 = 7 u	4x7 = 28 u	3x7 = 21 u	7 u + 28 u = 35 u

(a)
14 girls and 14 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 7 and 3 = 21 u

Percent of students who were girls at first
= ³⁰₃₉ x 100%
≈ 77% (Round off to whole number.)

Percent of students who are girls in the end
= ²⁸₃₅ x 100%
≈ 80% (Round off to whole number.)

Percent of students who were girls increased from 77% to 80%. (1)

(b)
Decrease in the number of girls
= 28 u - 30 u
= 2 u

2 u = 14

1 u = 14 ÷ 2 = 7

Number of students at first
= 39 u
= 39 x 7
= 273

Answer(s): (a) 1; (b) 273