how many two-digit numbers are there in wich the tens digit is greater than the one digit
if the ones is 0, there are 9
if the ones is 1, there are 8
if the ones is 2, there are 7
see a pattern?
....
if the ones is 9, there are 0
if the ones is 1, there are 8
if the ones is 2, there are 7
see a pattern?
....
if the ones is 9, there are 0