In the card game bridge, the 52 cards are dealt out equally to 4 players - called East, West, North, and South. If North and South have a total of 8 spades among them, what is the probability that East has 3 of the remaining 5 spades? The answer is (5C3)(21C10)/(26C13). However, I'm unclear where 21C10 comes from. Please explain this, especially the 10.