Choose which k of the n positions are heads, over all $2^n$ equally likely sequences:

$$P(\text{exactly } k \text{ heads}) = \frac{\binom{n}{k}}{2^n}$$

Return the probability as a float.

solve(3, 2)  ->  0.375      # HHT, HTH, THH out of 8
solve(5, 2)  ->  0.3125