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In a game of texas holdem, what are the odds that with a deck of 52 cards and 9 players are all playing at the table ( thats 2 cards each = 18 dealt to the players) would a complete royal flush be dealt to the board by the dealer?

 

I will assume that a royal flush is   J, Q, K, A    any suite.

 

The prob that is has a royal flush of spades      48C14   /    52C18       

This would be the same for each of the other suites too.   So you could just multiply this by 4

 

BUT

 

But there would be some double counting here.

 

The probability of getting any two royal flushes (say spades and diamonds) would be 44C10 / 52C18

This would have been added in twice so now it needs to be subtracted once

There are for suits 4C2 ways of choosing 2 suites = 6 ways

So I think we need to subtract    6*44C10/52C18

 

The prob of getting 3 royal flushes (say spades and diamonds and clubs) would be  40C6     /  52C18

There would be 4C3=4 ways of choosing  three suites.

Say you had S, D, C all  royal flushes.   

This hand would have been counted 3 times when counting singls flushes,  Then it would have been taken away 3 times when we were dealing with 2 royal flushes so it had better be added back in again now

There are 4C3=4 ways this can happen so we need to add back in    4* 40C6     /  52C18

 

Now there could be 4 royal flushes  there is only one way that this can happen.    36C2 / 52C18

This got added in 4 times when a single flush was looked at.

Then it go subtracted 6 times when 2 royal flushes were looked at

Then it got added 4 times when three royal flushes were looked at.

So it needs to be subtracted once.  because    4-6+4-1=1

 

 

4* 48C14 / 52C18   -   6*44C10/52C18     +   4* 40C6 / 52C18   -     36C2 / 52C18

 

(4* 48C14-6*44C10+4* 40C6-36C2) / 52C18

 

 

(4*nCr(48,14)-6*nCr(44,10)+4*nCr(40,6)-(36C2))/nCr(52,18)     =  approx   0.0449

 

The exact result - assuming that what I have done here is correct can be found here.

 

https://www.wolframalpha.com/input/?i=(4*nCr(48,14)-6*nCr(44,10)%2B4*nCr(40,6)-nCr(36,2))+++%2FnCr(52,18)

Mar 25, 2016
 #1
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Mar 25, 2016

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