PSLE
The number of boys and girls in a gymnasium were in the ratio 1 : 5. After 6 boys entered the gymnasium and half the number of girls left the gymnasium, the ratio of the number of boys to girls became 1 : 1. How many children were there in the gymnasium at first?

	boys	girls	Total
Before	1 u	5 u	6 u
Change	+ 6	- 2.5 u
After	1 u + 6	2.5 u

Number of girls who left the gymnasium
= 5 u ÷ 2
= 2.5 u

Number of boys in the end = 1 u + 6
Number of girls in the end = 2.5 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.

2.5 u = 1 u + 6
2.5 u - 1 u = 6
1.5 u = 6

1 u = 6 ÷ 1.5 = 4

Total number of children in the gymnasium at first
= 1 u + 5 u
= 6 u
= 6 x 4
= 24

Answer(s): 24