Keeping a String Secret in (Open) Source Code
You have at least two options, depending on what problem you want to solve.

If you want innocent readers of your code to not get the answers inadvertently, or you at least want to make it a bit difficult so that users are not tempted, you can encrypt the solutions and store the key as part of your code, perhaps a result of some computation (to make it even more difficult).

If you want to prevent users from retrieving the answer, you can use a one-way function, or in computer jargon, a hash function. Store a hash of the answer, and they you can test whether the answer is correct without it being possible to deduce the answer at all without finding it first. This has the disadvantage that it is harder to check for an answer that is close to the correct answer, though there are some solutions even to this problem.