system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the divisibility rule for 3 work?

If sum of all digits of any whole number in base ten is divisible by three, that number is divisible by three. Why does it work? Wikipedia gave me nothing, and the only explanation I found, was too complicated.
gpt
Take a three digit number ABC for example. The number is equal to 100A + 10B + C. You can rewrite this as 99A + 9B + (A + B + C). Now, 99A and 9B are obviously divisible by 3. So the entire sum is divisible by three if and only if (A + B + C) is divisible by 3.