1. I notice your solution uses recursion (at least I assume it uses recursion - I am not a ruby programmer). I think this would be an issue for large values of n (stack overflow).

2. I think an iterative function for fact would be more efficient.

3. I notice you do not memoize the result of fact even though you are calculating it twice for each call.

I suspect that you are trying to show how compact a solution can be, but in an interview situation, I would probably be more impressed by someone who is aware of the issues above.