Victor has four red socks, two blue socks, and two green socks in a drawer. He randomly picks two of the socks from the drawer, and is happy to see that they are a matching pair. What is the probability the pair was red?
There are $\left(\begin{array}{l}4 \\ 2\end{array}\right)=6$ possible red pairs, and then $\left(\begin{array}{l}2 \\ 2\end{array}\right)=1$ blue pairs, and finally $\left(\begin{array}{l}2 \\ 2\end{array}\right)=1$ green pairs.

So there were a total of $6+1+1=8$ possible pairs, of which there were 6 red ones. So the answer is $3 / 4$.