Discrete Mathematics, Mathematics, Combinatorics.
digit numbers that are even (and don’t start with 0) is 4. 2 digit: You have the bad guys: 22,44,66,88. So, how many even legitimate 2 digit numbers really are there? Total even 2 digits: 1st digit (non zero): 9. 2nd digit: 5 choices(0,2,4,6,8). 9x5=45 choices. Then, take total choices minus bad