The ratio of boys to girls in anya's class is 10 to 15. there are 20 boys how many girls are there

The correct answer would be: There are 30 girls in Anya's class. 

The result is this because the given ratio of boys to is 10 to 15. The number of boys is 20 which is 2 times the number given in the ration which means that to get to the number of girls in the class we have to multiply the ration of girls, in this case 15, with two. And this way we get to the number of 30 girls in Anya's class.

For more complicated numbers that cannot be calculated right away, always use the following equation:

A:B=C:x
A*x=B*C
x=B*C/A

Hope it helps :)

Really long apparently