/lit/ - Literature

>>19866647
Not sure if this is what you mean but here’s a screenshot of my undergrad course notes going over something similar.
Basically you can reduce any integer to a multiple of 9 plus a remainder by summing the digits until you just have one digit left (which is the remainder). In the example they use 2159483, which reduces to 5 modulo 9, meaning there’s an integer k such that 9k + 5 = 2159483.