# Probability and Poker

In the standard game of poker, each player gets 5 cards and places a bet, hoping his cards are "better" than the other players' hands.

The game is played with a pack containing 52 cards in 4 suits, consisting of:

13 hearts:
13 diamonds
13 clubs:

♥ 2 3 4 5 6 7 8 9 10 J Q K A
♦ 2 3 4 5 6 7 8 9 10 J Q K A
♣ 2 3 4 5 6 7 8 9 10 J Q K A
♠ 2 3 4 5 6 7 8 9 10 J Q K A

The number of different possible poker hands is found by counting the number of ways that 5 cards can be selected from 52 cards, where the order is not important. It is a combination, so we use C_r^n.

The number of possible poker hands

=C_5^52=(52!)/(5!xx47!)=2,598,960.

## Royal Flush

The best hand (because of the low probability that it will occur) is the royal flush, which consists of 10, J, Q, K, A of the same suit. There are only 4 ways of getting such a hand (because there are 4 suits), so the probability of being dealt a royal flush is

4/(2,598,960)=0.000\ 001\ 539

## Straight Flush

The next most valuable type of hand is a straight flush, which is 5 cards in order, all of the same suit.

For example, 2♣, 3♣, 4♣, 5♣, 6♣ is a straight flush.

For each suit there are 10 such straights (the one starting with Ace, the one starting with 2, the one starting with 3, ... through to the one starting at 10) and there are 4 suits, so there are 40 possible straight flushes.

The probability of being dealt a straight flush is

40/(2,598,960)=0.000\ 015\ 39

[Note: There is some overlap here since the straight flush starting at 10 is the same as the royal flush. So strictly there are 36 straight flushes (4 × 9) if we don't count the royal flush. The probability of getting a straight flush then is 36/2,598,960 = 0.00001385.]

The table below lists the number of possible ways that different types of hands can arise and their probability of occurrence.

## Ranking, Frequency and Probability of Poker Hands

 Hand No. of Ways Probability Description Royal Flush 4 0.000002 Ten, J, Q, K, A of one suit. Straight Flush 36 0.000015 A straight is 5 cards in order. (Excludes royal and straight flushes.) An example of a straight flush is: 5, 6, 7, 8, 9, all spades. Four of a Kind 624 0.000240 Example: 4 kings and any other card. Full House 3,744 0.001441 3 cards of one denominator and 2 cards of another. For example, 3 aces and 2 kings is a full house. Flush 5,108 0.001965 All 5 cards are from the same suit. (Excludes royal and straight flushes) For example, 2, 4, 5, 9, J (all hearts) is a flush. Straight 10,200 0.003925 The 5 cards are in order. (Excludes royal flush and straight flush) For example, 3, 4, 5, 6, 7 (any suit) is a straight. Three of a Kind 54,912 0.021129 Example: A hand with 3 aces, one J and one Q. Two Pairs 123,552 0.047539 Example: 3, 3, Q, Q, 5 One Pair 1,098,240 0.422569 Example: 10, 10, 4, 6, K Nothing 1,302,540 0.501177 Example: 3, 6, 8, 9, K (at least two different suits)

### Question

The probability for a full house is given above as 0.001441. Where does this come from?

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