Three digits number go from 100 to 999, and thus there are 999-100+1 = 900 of them.
The multiples of ten are numbers ending with zero, so we must exclude all numbers like xy0.
The first digit, x, range from 1 to 9, while the second digit, y, range from 0 to 9.
So, to generate a multiple of ten between 100 and 999, we have 9 choices for the first digit, 10 choices for the second, while the third digit is fixed to zero.
This means that there are multiples of ten between 100 and 999.
Since there are 900 numbers in total, there are 900-90 = 810 numbers between 100 and 999 that are not multiples of ten.