The ratio of girls to boys is 5:6.
Let the unknown number of girls be x.
If there are 15 boys in the same ratio, then the ratio of girls to boys is x:15.
Set the ratios equal as a proportion and solve for x.
5/6 = x/15
x = 15 * 5/6
x = 12.5
For the same ratio, you need 12.5 girls.
Since the number of girls must be a whole number,
there cannot be 15 boys.
