A box contains 12 marbles of three different colours: green, yellow and blue — 4 each. If you were to close your eyes and pick them at random, how many marbles must you take out to be sure that there are at least two of one colour among the marbles picked out?
Answer
You need to pick 4 marbles. In the first three pickings you may get 1 of each colour, on the 4th pick there must be at least two of one colour.