Final answer:
Person A's competitive equilibrium is achieved by allocating all their endowment to good x.
Step-by-step explanation:
In an exchange economy, for person A to have a competitive equilibrium, their utility should be maximized given their endowment and the prices of goods x and y. Since A's utility function is UA(x, y) = x + y, and they are endowed with 120 units of good x, their utility is maximized when they allocate all their endowment to good x. Therefore, xA = 120 units.