Matching pairs in the dark
You are packing a suitcase for a one day trip, and you need to bring a spare pair of socks. You have 30 single socks in your messy drawer, some are green, some are yellow and some are blue. Lights are off however, and you can't see what color is each sock you are picking. What is the minimum number of single socks you have to take from the drawer and put in your suitcase, to make sure that you have with you at least one pair of socks (matching colors)?
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1 solution
When you pick the first three socks, it is possible that one sock is yellow, one is blue, and one is green. However, when you pick the fourth sock, no matter what color it is, it makes a pair with one of the other socks.