PSLE
The number of boys and girls in a computer lab were in the ratio 1 : 3. After 10 boys entered the computer lab and half the number of girls left the computer lab, the ratio of the number of boys to girls became 1 : 1. How many children were there in the computer lab at first?

	boys	girls	Total
Before	1 u	3 u	4 u
Change	+ 10	- 1.5 u
After	1 u + 10	1.5 u

Number of girls who left the computer lab
= 3 u ÷ 2
= 1.5 u

Number of boys in the end = 1 u + 10
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 + 10
1.5 u - 1 u = 10
0.5 u = 10

1 u = 10 ÷ 0.5 = 20

Total number of children in the computer lab at first
= 1 u + 3 u
= 4 u
= 4 x 20
= 80

Answer(s): 80