Question

ther are 15 children are each wearing 1 cap .how many more caps woul you need to have one cap on each child ?

Answer 1

You would need 15 more, but other wise you already have a cap on each person, you don't need to add more unless you really are desperate. :D

Answer 2

You just started by saying that each child is wearing 1 cap. So you
already have one cap on each child, and you don't need any more.