Suppose that 10 cards, of which 5 are red and 5 are green, are placed at random in 10 envelopes, of which 5 are red and 5 are green. Determine the probability that exactly x envelopes will contain a card with a matching color

(x = 0, 1, 2, . , 10).

http://www.bilkent.edu.tr/~math250/Solution%20set1.pdf