If there are 5 girls and 6 boys, to find an equivalent ratio you could multiply both sides by, let's say, 2.
5x2:6x2 = 10:12
then you could do 3;
5x3:6x3 = 15:18
and finally 4.
5x4:6x4 = 20:24
you can multiply them by any number really, as long as you multiply them both by the same number.
