Answer:
a)
Crystal's opportunity cost to brew a gallon of root beer = 4/2 = 2 pizzas
Crystal's opportunity cost to make a pizza = 2/4 = 0.5 gallons of root beer
Hillary's opportunity cost to brew a gallon of root beer = 6/4 = 1.5 pizzas
Hillary's opportunity cost to make a pizza = 4/6 = 0.33 gallons of root beer
Crystal has the absolute advantage in making pizza (lowest time to make a pizza)
Crystal has the comparative advantage in making pizza (lowest opportunity cost).
b)
Crystal should trade pizza in exchange for root beer (she has the comparative advantage in making pizza)
c)
As long as the trading price of pizza is between 2/3 gallons of root beer (highest price) and 1/2 gallons of root beer (lowest price), both would benefit from the exchange. If the price of pizza is higher than 2/3 gallons of root beer, then Hillary will make her own pizza. If the price of pizza is lower than 1/2 gallons of root beer, then Crystal would brew her own root beer.