Please use the following information to answer the question. When the quantity of output is equal to 0 unit, the total cost is equal to $ 5 0 . When the quantity of output is equal to 1 unit, the total fixed cost is $ 5 0 and the total variable cost is $ 2 0 . When the quantity of output is equal to 2 units, the total cost is equal to $ 8 0 . What is average fixed cost equal to when quantity of output is equal to 2 units?