
Byron S. answered 11/04/14
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You can count the number of ways to choose a subgroup from a large group using the combination function:
nCx = n! / (n-x)! x!
There are 13C9 ways to select 9 clubs from the whole suit of 13 clubs. There are 52C9 ways to choose 9 cards from the entire deck.
52C9 / 13C9 is the number of ways of choosing 9 cards of a single suit from the deck.