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

A.) 181
B.) 165
C.) 121
D.) 99
E.) 44

Answer: It is given that M and N have the same digits in reverse order. Let M=10t+u and N=10u+t, where t and u are two digits. Then, M+N= (10t+u) + (10u+t) = 11(t+u). This means that any sum of the two integers M and N must also be a multiple of 11. Of the answer choices, only 181 is not a multiple of 11 and thus cannot be the sum of M and N

HELP!!!! I do not understand where the author came up with the number 10. Why 10t and 10u? please help, I understand that 181 is not a multiple of 11 and the rest are, hence the correct answer: but I do not understand the reasoning!

t and u are single digits 0-9, so 10t + u makes a 2 digit number,

ie. if t=2 and u=4, 10t + u=24