PSLE
The number of boys and girls in a music room were in the ratio 1 : 5. After 9 boys entered the music room and half the number of girls left the music room, the ratio of the number of boys to girls became 1 : 1. How many children were there in the music room at first?

	boys	girls	Total
Before	1 u	5 u	6 u
Change	+ 9	- 2.5 u
After	1 u + 9	2.5 u

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

Number of boys in the end = 1 u + 9
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 + 9
2.5 u - 1 u = 9
1.5 u = 9

1 u = 9 ÷ 1.5 = 6

Total number of children in the music room at first
= 1 u + 5 u
= 6 u
= 6 x 6
= 36

Answer(s): 36