In a village, there are 87 families of which 52 families have at most 2 children. In a rural development programme, 20 families are to be helped chosen for assistance, of which at least 18 families must have at most 2 children. In how many ways can the choice be made?

Answer

`=""^(52)C_(18)xx ""^(35)C_(2)+ ""^(52)C_(19)xx ""^(35)C_(1)+ ""^(52)C_(20)`.