Black and white socks

There are 20 black socks and some white socks in a drawer. Can you work out how many of the socks are white?
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There are 20 black socks and $n$ white socks in a drawer.

When a sock is taken from the drawer, the probability that it is white is $\dfrac{1}{n}$.

What is the value of $n$?

This problem is adapted from the World Mathematics Championships