>>14198753You have 10 elements 0-9. These elements loop in a wheel as the scale increases. There are patterns to the sums of these digits dependent on the one's place. Also, reduction of multiple digits to a singular digit gives you a check and balance on the final answer in addition and multiplication. One more thing of note is that, in addition, 9 is like a phantom digit, but in multiplication, it's a contagion. 3 and 6 are arms of it. The primes are locked multiplicatively by 3,6, and 9, as such the sum of their digits will always be solely 3 or 6.
