Boys to girls
3 : 5
? : 35
35/5 = 7
5 x 7 gives number of girls.
So, 3 x 7 will give number of boys, which is 21
Thus, there are 21 boys
(basically, you find the number you multiplied by 5 to get 35, then, multiplying that number on the other side, which was 3 in this case. )
Hope this helps!