You must find a number that will divide into the total of the girls and boys and come up with an even number. By looking at the numbers, you can see that both 56 and 91 will divide evenly by 7. The common denominator for both calculations is 7.
56/7=8 girls
91/7= 13 boys
Add 8 girls and 13 boys and you will have 21 in each group. That way, all groups are equally boys and girls.
