Since we can add up the factors of 28 (excluding 28 itself) to get 1+2+4+7+14 = 28 this means that 28 is a perfect number. In the interval from 20 to 30, it is the only perfect number.
Answer: 28
Note: Something like 22 is not a perfect number since 1+2+11 = 14 which isn't 22 like we want. Every number, in the interval from 20 to 30, but 28 has this problem.
