75 is a self number because there is no integer that added up to its own digits adds up to 75. A self number in a given number base b is a natural number that cannot be written as the sum of any other natural number n and the individual digits of n. 20 is a self number (in base 10), because no such combination can be found (all n < 15 give a result less than 20; all other n give a result greater than 20). 21 is not, because it can be written as 15 + 1 + 5 using n = 15.