the problem and answer can be found here I'd hope that you try to solve it without looking at the answer.
http://www.inficad.com/~ecollins/logic1-10.htm

Tom is from the census bureau and greets Mary at her door. The have the following conversation:

Tom: I need to know how old your three kids are.
Mary: The product of their ages is 36.
Tom: I still don't know their ages.
Mary: The sum of their ages is the same as my house number.
Tom: I still don't know their ages.
Mary: The younger two are twins.
Tom: Now I know their ages! Thanks!