There are two ways to figure this out. Both ways are correct.
1. Fractions
9/7=x/63
from here you cross multiply
7x=9×63
7x=567
now to get x by itself you divide by 7
x=567÷7
x=81
2. If you have 63 girls and 7 girls to each group. You can divide 63 by 7 and find out how many groups you have which is 9. 9 groups, 7 girls each. For every 7 girls there are 9 boys. 9×9 ( 9 groups, 9 boys each)
