* When we multiply three numbers, we will get the prime numbers.
* The difference between the second and the first number is equal to the third and second.
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?