In this problem, notice that $a + b + c$ is going to be the maximum of the given $4$ numbers. For each of the other $3$ numbers, subtracting it from the maximum value gives you one of the desired $a$, $b$, or $c$. The answer cares not about the order of $a$, $b$, and $c$, all of which are positive integers and of course $(a + b + c) - (a + b + c) = 0$.