Find the Number of Positive 2-Digit Odd Numbers With Distinct Digits
1 2 3 4 5 6 7 8 9, first 9 digits
1 3 5 7 9, first 5 odd digits
45=9*5 because
11 13 15 17 19
21 23 25 27 29
and so on, all the way down to 90s
then remove 11, 33, 55, 77, 99
45-5=40