A woman lives in a skyscraper thirty-six floors high and is 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?
What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?