system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: A lot people know that if the sum of the digits of a number is a multiple of 3, then the number is also a multiple of three. How can this be explained mathematically?
gpt
If you've got a three digit number "abc" then you can write it as    
100a + 10b + c

You can rewrite that as    
99a + 9b + a + b + c

The 99a+9b part is definitely a multiple of 3, so the whole number will be a multiple of 3 as long as (a+b+c) is.

Nothing we did depended on having 3 digits, so the same argument will work for any number of digits.