How many two-digit numbers exist such that the tens digit is larger than the ones digit?
If the tens digit is 9 then it is 9 0,1,2,3,4,5,6,7,8
If the tens digit is 8 then it is 8
..
If the tens digit is 0 then it is 0
9+8+7+6+5+4+3+2+1 =
9+8+7+6+5+4+3+2+1 = = 45
