Discrete Mathematics, Mathematics, Combinatorics.
1–9, so if its not zero, you lose one of your options. This tells you that the answer is between 10 x 9 x 9 x 8 and 10 x 9 x 9 x 9 How many four digit numbers (not starting with 0) are even and have no 2 same digits next to each other? 9 choices for the first three digits, but for the fourth digit