Answers

2016-01-02T21:59:48+08:00
The answer is 15:25 since
 3+5 = 8
40/ 8 = 5
3 x 5 = 15
5 x 5 = 21
Although, I am not sure of my solution, I am perfectly sure with my answer, And I cannot tell which one of in the ratio is the number of boys nor girls since your question lacks information.
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I'm sure it's just a typo error, and your answer to 5 x 5 = 25, and not 21 :-)
2016-01-03T09:06:53+08:00
This is a kind of a problem that includes partitive proportion. 
3 + 5 = 8
40 ÷ 8 = 5

5 × 3 = 15
5 × 5 = 25

Therefore, the ratio will become 15:25 = 3:5

Check:
15 + 25 = 40

Also, your problem is incomplete but it's okay.. we have no ideas if the ratio of 3:5 is the ratio of boys to girls or girls to boys.
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