Combinatorics and probability
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A club of nine people wants to choose a board of three officers, President, Vice President, and Secretary. Assuming the officers are chosen at random, what is the probability that the officers are Marcia for president, Sabita for Vice President, and Robert for Secretary? So to think about the probability of Marcia-- so let me write this-- President is equal to Marcia, or Vice President is equal to Sabita, and Secretary is equal to Robert. This, right here, is one possible outcome, one specific outcome. So it's one outcome out of the total number of outcomes, over the total number of possibilities. Now what is the total number of possibilities? Well to think about that, let's just think about the three positions. You have President, you have Vice President, and you have Secretary. Now let's just assume that we're going to fill the slot of President first. We don't have to do President first, but we're just going to pick here. So if we're just picking President first, we haven't assigned anyone to any officers just yet, so we have nine people to choose from. So there are nine possibilities here. Now, when we go to selecting our Vice President, we would have already assigned one person to the President. So we only have eight people to pick from. And when we assign our Secretary, we would've already assigned our President and Vice President, so we're only going to have seven people to pick from. So the total permutations here or the total number of possibilities, or the total number of ways, to pick President, Vice President, and Secretary from nine people, is going to be 9 times 8 times 7. Which is, let's see, 9 times 8 is 72. 72 times 7, 2 times 7 is 14, 7 times 7 is 49 plus 1 is 50. So there's 504 possibilities. So to answer the question, the probability of Marcia being President, Sabita being Vice President, and Robert being Secretary is 1 over the total number of possibilities, which is 1 over 504. That's the probability.