A woman lives in a skyscraper thirty-six floors high and served by several elevators which stop at each floor going up and down. Each morning she leaves her apartment and goes to one of the elevators. Whichever one she takes is three times more likely to be going up than down. Why?
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She lives on the 27th floor. The elevator came down from the 36th to the 28th floor – 9 floors; or it came up from the 1st to the 27th floor – 27 floors. Therefore there is a 3 to 1 chance of it going up rather than down.