Your sock drawer contains ten pairs of white socks and ten pairs of black socks. If you're only allowed to take one sock from the drawer at a time and you can't see what color sock you're taking until you've taken it, how many socks do you have to take before you're guaranteed to have at least one matching pair? you can say at the worst case.

- At most it will take three taking. At the first two trials, if your shocks appear of two different colours, the third one will definitely match any one of the previous two.