In: Statistics and Probability
You draw two cards from a standard deck without replacement and without looking at them. Then the third card is drawn and it is revealed: it is the ace of hearts. What is the conditional probability that your two cards, formerly hidden, are aces?
There are 4 aces and 48 other cards in a deck.
There are three cases possible :
First two card are aces or one of first two cards is ace or none of first two card is ace.
Case: First two card are aces
The probabilty of getting third ace and first two aces is
P(first two aces and third ace) =(4/52) * (3/51) * (2/50) = 24 /(52*51*50)
Case: One of first two cards is ace
The probabilty of getting third ace is
P(one of first two is an ace and third ace) =2 * (4/52) * (48/51) * (3/50) = 1152 / (52*51*50)
Case: None of first two cards is ace
The probabilty of getting third ace is
P(none of first two is an ace and third ace) = (48/52) * (47/51) * (3/50) = 6768 / (52*51*50)
The probability that third card is an ace is:
P(third ace) = [24 /(52*51*50)]+ [1152 / (52*51*50)] + [9024 / (52*51*50)] = 10200 / (52*51*50)
The conditional probability that your two cards, formerly hidden, are aces is
P(first two cards ace |third ace) = P(first two aces and third ace) / P(third ace) = 24 / 10200 = 0.0024