A neat way to improve on this would be to get a series of 3-5 challenges, especially if it's something like math and logic puzzles that build on each other, so it's a multi-dimensional problem, so it's difficult to cache or precompute.

The hash of the answer is just a string that concatenates the answers, and the challenges always mixes them in different orders. One possible example:

1. Does Red combine with Yellow to make Green or Orange? 2. If the answer to the last question was reversed, what letter would it start with? 3. If you added that latter to the end of these words, which word would be most edible? Mac, Nam, Pi, Snak

answers: orange,e,pi

Of course, you'd want to design the interface to support multiple choice selection.

I would also recommend against using MD5, since, even if the hash weren't known to the end user, which should be sufficient even in this case, the attack MD5 is most known for is the fact that it's trivial to generate text that could match any given hash, regardless of what was used to originally make it. It seems like a potential attack vector somehow, depending on the case, and it's not terribly harder to just use one of the many tried-and-true, known-not-broken cryptographic hashing algorithms. SHA-256 would be adequate.

I'm not 100% certain of all the logic behind this, there's always cases I'm not rigorous to consider, but I'd be interested in seeing how others might improve this approach in similar ways.