So inspired by a recent CMC in TNB, I have a question that maybe you math nerds/geniuses could help with. 3 and 9 have interesting modular properties in base 10. The sum of the digits of any factor of 3 or 9 will always be another factor of 3 or 9. Now it's pretty obvious why this works for 9, because in base b=10
, 9 is b-1
. And this works for other bases (multiples of 4 in base 5 sum to factors of 4 for example).