PSLE The number of boys and girls in a classroom were in the ratio 3 : 7. After 3.5 boys entered the classroom and half the number of girls left the classroom, the ratio of the number of boys to girls became 1 : 1. How many children were there in the classroom at first?
|
boys |
girls |
Total |
Before |
3 u |
7 u |
10 u |
Change |
+ 3.5 |
- 3.5 u |
|
After |
3 u + 3.5 |
3.5 u |
|
Number of girls who left the classroom
= 7 u ÷ 2
= 3.5 u
Number of boys in the end = 3 u + 3.5
Number of girls in the end = 3.5 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.
3.5 u = 3 u + 3.5
3.5 u - 3 u = 3.5
0.5 u = 3.5
1 u = 3.5 ÷ 0.5 = 7
Total number of children in the classroom at first
= 3 u + 7 u
= 10 u
= 10 x 7
= 70
Answer(s): 70