PSLE The number of boys and girls in a computer lab were in the ratio 2 : 5. After 5 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 |
2 u |
5 u |
7 u |
Change |
+ 5 |
- 2.5 u |
|
After |
2 u + 5 |
2.5 u |
|
Number of girls who left the computer lab
= 5 u ÷ 2
= 2.5 u
Number of boys in the end = 2 u + 5
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 = 2 u + 5
2.5 u - 2 u = 5
0.5 u = 5
1 u = 5 ÷ 0.5 = 10
Total number of children in the computer lab at first
= 2 u + 5 u
= 7 u
= 7 x 10
= 70
Answer(s): 70