**Quote:**

because I read an information, sound likes if girls is A, boys is B => A/B = 4/3.

You use the classical ratio approach. A : B = 4 : 3. That's completely fine and works here well.

Note, that you get the same result. A – B = 20 – 15 = 5 more girls than boys.

**Quote:**

I do not understand why the explanation can use ratio 3/7 and 4/7.

How do we get 3/7 and 4/7 so fast, knowing the ratio 3 : 4?

The ratio 3 : 4 gives us:

boys = 3

*x*, girls = 4

*x*. The total number of students is 3

*x* + 4

*x* = 7

*x*So boys make 3

*x*/7

*x* = 3/7. Girls make 4

*x*/7

*x* = 4/7.

So if we know that some total set is composed of just A and B in ratio

*a* :

*b*, then A makes

*a*/(

*a* +

*b*) and B makes

*b*/(

*a* +

*b*) of the whole.

Furthermore, it is true for any number of subset objects. Just make sure they all compose the whole set. For example,

If an alloy is composed of three elements in ratio 2 : 3 : 4, then

element 1 is 2/9 of the alloy

element 2 is 3/9 of the alloy

element 3 is 4/9 of the alloy