If a, b, c are positive integers less than 13 such that
2ab + bc + ca = 0 (mod 13)
ab + 2bc + ca = 2abc (mod 13)
ab + bc + 2ca = 3abc (mod 13)
then determine the remainder when a + b + c is divided by 13.
See the solution here: https://web2.0calc.com/questions/modular-number-theory#r6
