5% is 1/20. If the number of boys is b then the number of girls is b/20 more, i.e., 21b/20 (b+b/20). The total number of girls and boys is b+21b/20=82. Multiply throughout by 20 to get rid of the fraction: 20b+21b=20*82. So 41b=20*82. Divide throughout by 41 and b=40. So the number of boys is 40 and the number of girls another 5% of this (1/20) making 42.