I am really curious how do you know this?
Looking good in this corner of the thread!
I learnt it first as a math trick for children in Vietnam: to know if something is divisible by 9, you calculate its digitial root and see if the digital root is divisible by 9 (same goes for 3).
When I grew up, I was curious and tried to prove that mathematically, which is how I ended up with digitalRoot(num) = num % 9 (or 9 if divisible by 9) :D
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.