If the two-digit integers M and N are positive and
have the same digits, but in reverse order, which of
the following CANNOT be the sum of M and N ?
1st integer = MN = 10M + N
2nd integer = NM = 10N + M
Sum = 10M + N + 10N + M = 11M + 11N = 11 (M+N)
Therefore, sum of M+N is a multiple of 11. Anything that isn't a multiple of 11 in the answer choices is out answer. 181 is it.