PSLE The number of girls and boys in a computer lab were in the ratio 2 : 7. After 15 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 |
2 u |
7 u |
9 u |
Change |
+ 15 |
- 3.5 u |
|
After |
2 u + 15 |
3.5 u |
|
Number of boys who left the computer lab
= 7 u ÷ 2
= 3.5 u
Number of girls in the end = 2 u + 15
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 = 2 u + 15
3.5 u - 2 u = 15
1.5 u = 15
1 u = 15 ÷ 1.5 = 10
Total number of children in the computer lab at first
= 2 u + 7 u
= 9 u
= 9 x 10
= 90
Answer(s): 90