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)