Successfully pasing a code challenge is not a good predictor of future success. However, failing to pass a code challenge is a very good predictor of future failure. Furthermore, making the test harder does not make it predict future success well.

Therefore, I belong to the "FizzBuzz" school: Make the test ridiculously simple. It should take no more than fifteen minutes to complete. If the person gets it obviously right, throw the result away and move on to more important quetsions. Do not try to deduce a lot of malarky about their programming style or ability from an obviously contrived problem.

For example, writing:

a = 5; b = 10

Does not mean they don't know Ruby well, and writing:

a, b = 5, 10

Does not mean they are a Ruby expert. And writing:

fsquare = lambda { |x| x * x }

Does not mean they are a closet Bipolar Lisp Programmer. If the program works, it's a pass, move on. If there is a syntax error or some such, who cares, move on.

But if they struggle... You need to investigate the reason for their difficulty with a simple problem.