Cut numbers in {1..9} into 2 parts, and group them into two dec numbers, a and b, respectively.
Given k∈{2..9}, and for different k, look for all possible pairs of a and b that meets the condition a*k == b
For every dec c, c*k can only be one digit longer or no longer than c. Since the toal length of a and b (in dec) has to be 9, we can get know that b has to be 5 digits long while a has to be 4 digits long. This makes the code a lot simpler.
For every k in {2..2..8}, we have b.lastDigit % 2 == 0. And for every k in {3..3..9}, we have b.allDigitSum % 3 == 0. Now we have the following code for listing everything we need.