Inclusively, there are 99 integers from 1 through 99.  9 of these are 
1-digit integers, so that leaves 99-9 or 90 2-digit integers inclusively 
from 10 through 99. Tack on a 7 at the right end of each these 90 
two-digit integers and we have the desired set of 3-digit integers with 
a 7 as the units digit..

Answer: 90

Edwin