The greatest common divisor of 21 and some number between 50 and 60 is 7. What is the number?
I can start by listing the multiples of 7 in that range: 49, 56, 63. 49 is too low and 63 is too high, so 56 is the only possible candidate. To check, I can divide 56 by 7 and get 8, which has no common factors with 21 except 1. Therefore, 56 is the number I'm looking for. 