Question 382715
There are nine digits (0-9) for the hundreds' digit, and 10 choices for the tens' and units'. This leaves 9*10*10 = 900.

Or, we can count the number of three digit numbers in the base-10 counting system. It ranges from 100 to 999, so the number of elements in the set {100, 101, ..., 999} is 900 (since the set is inclusive).