I heard this one many years ago. I have rewritten it, to make it harder and to avoid that my dear readers search the Internet for the answer instead of thinking themselves.
A mathematician, Professor Anderson, visited her colleague Dr. Smith. During the visit Anderson asked: “How old are your three kids actually?”
Dr. Smith thought for a moment, and then he answered: “None of them have the same age, the sum of their ages is less than eleven and the product of their ages is a number you know well, namely the street number of my house”.
This made Professor Anderson think, but after a short moment she said: “It can’t be solved, since we are missing…”
Dr. Smith interrupted, almost laughing: “Sure, we are…” and then he gave Professor Anderson an extra piece of information.
Now they both knew the exact ages of Dr. Smith’s children.
What is the street number of Dr. Smith’s house?