Explanation:
Bill will benefit from trade If 1P > 3C and Fred will benefit from trade If 1P < 5C
Thus, both will benefit from exchange if 3C < 1P < 5C.
That means that both of them would benefit from trading if 1 pie are to be traded for more than 3 cakes and less than 5 cakes like 1 pie is exchanged for 4 cakes. (As a result, since both sides are to profit from exchange, we should expect 1 pie to be exchanged for 4 cakes)