The units digit or the ones digit of an integer is the rightmost digit. For example the integer 564 has ones digit 4. I think this question is asking if you have 2 two digit numbers what is the largest value possible for th sum of the ones digits. Since the digits are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 the largest the digits can be is 9 so the largest possible sum is 9 + 9 = 18.

Penny