x = both clubs Club B has 25 - x...u take the total and subtract the ones that are in both Club A has 17 - x... same here ^
x + (25 - x) + (17 - x) = 38...so both + club B + club A = total -2x + x + 42 = 38 -x = 38 - 42 -x = - 4 x = 4...so there are 4 members that belong to both clubs <==
so there are 17 - 4 = 13 in club A only and 25 - 4 = 21 in club B only 4 in both