PSLE
The number of boys and girls in a school hall were in the ratio 1 : 3. After 8 boys entered the school hall and half the number of girls left the school hall, the ratio of the number of boys to girls became 1 : 1. How many children were there in the school hall at first?

	boys	girls	Total
Before	1 u	3 u	4 u
Change	+ 8	- 1.5 u
After	1 u + 8	1.5 u

Number of girls who left the school hall
= 3 u ÷ 2
= 1.5 u

Number of boys in the end = 1 u + 8
Number of girls in the end = 1.5 u

Since the ratio of the number of boys to girls in the end is 1 : 1, the number of boys and girls in the end is equal.

1.5 u = 1 u + 8
1.5 u - 1 u = 8
0.5 u = 8

1 u = 8 ÷ 0.5 = 16

Total number of children in the school hall at first
= 1 u + 3 u
= 4 u
= 4 x 16
= 64

Answer(s): 64