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Bunuel
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MartyMurray
Peter took a 52-card deck of cards and removed all small cards, all spades, and all clubs. The only remaining cards were 10, Jack, Queen, King, and Ace in the suits of heart and diamond. Of these 10 cards, he deals himself 5 cards. What is the probability that Peter deals himself at least one pair of cards?

A. 7/10
B. 17/33
C. 55/63
D. 66/165
E. 18/25


Peter is choosing 5 cards from 10.

10 x 9 x 8 x 7 x 6/5! = 9 x 4 x 7

This will be the denominator in our answer.

A, D, and E have multiples of 5 in their denominators. 5 is not among the factors of our denominator.

B has a multiple of 11 in the denominator. 11 is not among the factors of our denominator.

The only possible answer is C.

--------------------------------------------------------------------------------------------------

Alternative approach:

9 x 4 x 7 = 252

The number of ways to select 5 cards without including any pairs is 10 x 8 x 6 x 4 x 2/5! = 32

The number of ways to select 5 cards with at least one pair is 252 - 32 = 220.

220/252 = 55/63

The correct answer is C.
Hey marty, could you explain why you've divided the number of ways to select 5 cards w/o any pairs by 5!? I got the 10 x 8 x 6 x 4 x 2 bit but didn't quite understand why we need to divide by 5!?
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we can do via negation -
firstly 10C5 = 252
1 - no pairs (all unique cards)
so no pairs will be 2c1 * 2c1 *2c1 *2c1 *2c1 - one card from each pair so total = 32

ans = 1- 32/252 = 55/63
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