Why don't you give them your answer and state the fact that you used whatever you did?

This way you're not fooling anyone, you're showing you have ethics and, something that people quite often miss, you're showing that you can find other solutions that were not created by you.

It's kind of the same advantage of hiring someone that will always look on Perlmonks to see if someone did the task before, or someone that will always look for an open source project that will solve his problem before trying to solve it him/herself. It's actually a good thing!