In this case, c = (a + b)/2.
In order to solve this, set up the equation and solve by cross multiplying.
a+b+c/3 = a+b/2 ----> Cross multiply.
3(a + b) = 2(a + b + c)
3a + 3b = 2a + 2b + 2c ----> Subtract 2a from both sides
a + 3b = 2b + 2c -----> Subtract 2b from both sides
a + b = 2c -----> Divide both sides by 2
(a + b)/2 = c