How many numbers are there between 500 and 600 in which 9 occurs only once?
The first digit is 5.
5 _ _
There are 2 cases.
5 9 a, where a is not 9, or 5 b 9, where b is not 9.
9 options for a, 9 options for b. 9 + 9 = 18
=^._.^=
