jonesthecurl wrote:Looks like a 1 in 7.
Possibilities are:
Spade/Spade
Spade/not
Spade/not
Spade/not
not/Spade
not/Spade
not/Spade.
Correct, but restated a little easier to follow...
For the experiment, there are 4 cases.
Case A: spade/spade Probability 1/4 x 1/4 = 1/16
Case B: spade/not spade Probability 1/4 x 3/4 = 3/16
Case C: not-spade/spade Probability 3/4 x 1/4 = 3/16
Case D: not-spade/not-spade Probability 3/4 x 3/4 = 9/16
The question is "If one of the two cards was a spade, what is the probability that the other one also was?" If you rephrase the question, "what is the probability that both cards are spades, given that at least one card is a spade?" you have a classic question of conditional probability and the answer is (1/16)/(1/16+3/16+3/16)= 1/7.
Note since the draws are independant, a nitpicker could argue that that the original question is better answered by the 1/4 answer. Although the question doesn't specify whether the first draw or the second is the given spade, it does specify that you are looking at the probability that the OTHER draw is spade- thus you are looking at either Case A and Case B or at Case A and Case C, but either way your conditional probability is back to 1/4.
Related riddle- Monty Hall paradox.
You are on the game show, Let's Make A Deal. The host shows you three doors. Behind one of the doors is an expensive convertible, but behind the other two are various farm animals. You are asked to pick one of the doors, and you win whatever is behind it. You pick your door. Befort the host opens the door you picked, he opens one of the doors you did not pick, revealing an incontinent goat. (Ewg!) He then offers to trade you, your door and the contents it would reveal for the other unopened door and its contents. Assuming that you would rather have a convertible then a farm animal, should you make the trade, reject the trade, or does it matter?