Given a bit string $x$ of length $n$, how many strings are there that differ from $x$ in exactly $k$ positions?
Also, if we replaced the case with decimal strings, how would it change?
I'm a bit stuck. I'm not sure which method to use to solve this question. I'm between whether to use regular permutation, or difference method, but both of those could still be incorrect.
Any insight would be helpful, thank you!