This is just a matter of probability. Let's look at the shoes first. He only has two pairs, so if he chooses one at random, there is a 50% chance it will be brown and 50% it will be black. 50% he will choose the shoes asked in the problem. Now let's look at the socks. The problem asks if he wears any 3 of 5 socks, so there is a 60% he will choose one of the socks asked in the problem.
We can write this out as fractions: 1/2 - shoes. 3/5 - socks. To get them together, you need to multiply them together.
1/2 * 3/5 = 3/10. So the answer is 30%
