Black and white socks

Answer: 5

There are $20+n$ socks in the drawer altogether, so $\dfrac{n}{20+n}=\dfrac{1}{n}$.

$$\begin{array}{rl}& \frac{n}{20+n}=\frac{1}{n}\\ & \Rightarrow n=\frac{1(20+n)}{n}\\ & \Rightarrow n^2=20+n\\ & \Rightarrow n^2-n-20=0\\ & \Rightarrow (n+4)(n-5)=0\\ & \Rightarrow n+4=0\text{or}n-5=0\\ & \Rightarrow n=-4\text{or}n=5\end{array}$$ $n$ must be positive as there cannot be a negative number of white socks, therefore there are $5$ white socks in the drawer.