If we tell you that there is a relation between the numbers and letters in the given figure, can you analyze it and find the missing letter in the last box?
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?