PSLE At first, the ratio of the number of girls to the number of boys in a kindergarten was 2 : 7. After 9 boys and 9 girls joined the kindergarten, the ratio became 1 : 2.
- Did the percentage of pupils who were boys increase (1), decrease (2) or remain the same (3)? Give your answer in number. (Eg Increase = 1)
- In the end, how many pupils were there in the kindergarten?
|
Girls |
Boys |
Difference |
Total |
Before |
2x1 = 2 u
|
7x1 = 7 u |
5x1 = 5 u
|
2 u + 7 u = 9 u |
Change |
+ 9 |
+ 9 |
|
|
After |
1x5 = 5 u |
2x5 = 10 u |
1x5 = 5 u |
5 u + 10 u = 15 u |
(a)
9 boys and 9 girls joined the kindergarten. There was equal number of boys and girls who joined the kindergarten.
Since the increase in number of pupils in both groups is the same, the difference between the number of boys and girls remains unchanged.
We make the difference between the number of boys and girls at first and in the end the same.
LCM of 5 and 1 = 5 u
Percent of pupils who were boys at first
=
79 x 100%
≈ 78% (Round off to whole number.)
Percent of pupils who are girls in the end
=
1015 x 100%
≈ 67% (Round off to whole number.)
Percent of pupils who were boys
decreased from 78% to 67%. (2)
(b)
Increase in the number of boys
= 10 u - 7 u
= 3 u
3 u = 9
1 u = 9 ÷ 3 = 3
Number of pupils in the end
= 15 u
= 15 x 3
= 45
Answer(s): (a) 2; (b) 45