Answer:
Casey's opportunity cost of producing 1 kg of potatoes is 5 kg of steak.
Casey's opportunity cost of producing 1 kg of steak is 0.2 kg of potatoes.
Rick's opportunity cost of producing 1 kg of potatoes is 3 kg of steak.
Rick's opportunity cost of producing 1 kg of steak is 0.33 kg of potatoes.
Casey should produce steak while Rick should produce potatoes, since Rick has a comparative advantage in producing potatoes (lower opportunity cost) and Casey has a comparative advantage in producing steak.
As long as the price of steak per kilogram of potatoes is less than 5 kg of steak and more than 3 kg of steak, then both would win. In order for both of them to win is a similarly proportional way, the exchange price should be 4 kg of steak per kg of potatoes.