30 girls and boys have planned for a picnic. there is a of 3 girls to 7 boys. how many boys are there?

Out of 10 children, 7 are boys. So, since we have 3 sets of 10 children, these 3 sets must have 3 X 7 = 21 boys.

You can set up a proportion:  30 girls / n boys  =  3 girls / 7 boys

Cross-multiply:  3 x n  =  30 x 7

                           3n  =  210

                             n  =  70