PSLE
The number of girls and boys in a computer lab were in the ratio 3 : 7. After 5 girls entered the computer lab and half the number of boys left the computer lab, the ratio of the number of girls to boys became 1 : 1. How many children were there in the computer lab at first?

	girls	boys	Total
Before	3 u	7 u	10 u
Change	+ 5	- 3.5 u
After	3 u + 5	3.5 u

Number of boys who left the computer lab
= 7 u ÷ 2
= 3.5 u

Number of girls in the end = 3 u + 5
Number of boys in the end = 3.5 u

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

3.5 u = 3 u + 5
3.5 u - 3 u = 5
0.5 u = 5

1 u = 5 ÷ 0.5 = 10

Total number of children in the computer lab at first
= 3 u + 7 u
= 10 u
= 10 x 10
= 100

Answer(s): 100