The sum of digits of the number 8215 is 8 + 2 +1 +5 = 7 (mod 9)

Observe also 8215 = 9 9912) + 7= 7 (mod 9) does this hold for any number? Explain

Suppose we want to compute the product 8215 x 3567 modulo 9. Replacing these numbers by those obtained by adding their digits and reducting modulo 9 gives 16 x 21 = 7 x 3 =21 = 3 (mod 9). Is it true that 8215 x 3567 = 3 (mod 9)? Explain