Discrete Mathematics, Mathematics, Combinatorics.
you need to be even. However, if the previous digit was even, then you only have 4 choices for the last digit. So, the answer is between 9³ x 4 …. 9³ x 5 …but what is it exactly? Let’s start from the bare bones.(even and no 2 digits next to each other) 1 digit version: (even)- The number of one