Redstone Math Toolbox Part 1: Modular Arithmetic

[Zijkhal]

    To introduce the topic, I would like to give the reader a nice trick I've used many times in my life. If you want to tell if a number is divisible by 3, add up the digits and if the result is divisible by 3, then so is the original number (the same property applies to 6...

I dont think so. 18 is divisible by 6, but the sum of its digits, 9 isnt. The division rule for 6 is if its divisible by both 3 and 2, then its divisible by 6, using the rule that if b = c*d, c and d are relative primes, then a mod b = 0 if and only if a mod c = 0 AND a mod d = 0.

Or if we replace 3 in your proof with 6, we get 4^n mod 6, so it does matter which digit is which, so we cant simply sum them to determine divisibility. Based on this (unless I'm missing something), we cant create a rule for divisibility for any number based solely on the sum of its digits. (for 6, its more like a0 + a1*4 + a2*16 + ... + an*4^n, which is a smaller number that what we begun with, but not as convenient as simply summing the digits)

Apart from that, I think its good, but I would add a statement at the start of the modular arithmetic part (for beginners to the topic) that modular arithmetic deals with whole numbers, so all numbers (in the modular arithmetic segment / chapter) from now on are to be assumed whole, unless stated otherwise.

He is correct on the 3s, I think he meant to put 9 not 6.
(Only works for 3 and 9).. if digits sum to a multiple of 3 or 9, that number is divisible by 3 or 9 respectively. 
Called "Rule of 3 and 9" or "Division Rules"

But he said 6 AND 9, I only cut the 9 part