Answer: 81
Step-by-step explanation:
The two digit counting numbers start at 10 and end at 99.
{10, 11, 12, ..., 98, 99}
There are m = 99-10+1 = 90 items in this list.
The two-digit multiples of 10 are: 10, 20, 30, 40, 50, 60, 70, 80, 90
There are n = 9 items listed here. It might help to think of the set {1,2,3,4,5,6,7,8,9} and tack 0's at the end of each.
So we have m-n = 90-9 = 81 two-digit counting numbers that are not multiples of 10.