Sample Space Reduction

Let $B$ be a non-empty set with $|B| = n$, and let $A \subseteq B$ with $|A| = k$, where $0 \leq k \leq n$. A uniformly random element of $B$ is selected. For any $b \in B\setminus A$, compute the probability $b$ is selected given that the chosen element is not in $A$. Solve this when $n = 10$ and $k = 4$.