Find the remainder when 1^2 + 2^2 + 3^2 + ... + 50^2 is divided by 15.
Sum of the first n squares = n (n + 1) (2n + 1) / 6
Sum of the first 50 squares = 50 ( 51) ( 101) / 6 = 42925
42925 mod 15 = 10
