Answer:
6 Cookies {or any >5, <7)
Step-by-step explanation:
Theory of Comparative Advantage states : A person/ economy having lesser opportunity cost (i.e other good sacrifised) to attain a good, should sell it to - other person/ economy having the good's higher opportunity cost.
Trade is beneficial if the terms of trade exchange ratio is better than own account production sacrifise ratio.
Bill can bake a pie with opportunity cost of 5 cookies. Fred can bake a pie with opportunity cost of 7 cookies. Bill has less opportunity cost of Pie in terms of Cookies, so should sell it to Fred.
The trade between them will be beneficial only if : both of them gain from trade - i.e get a good at lower opportunity cost than their own. Fred getting 1 pie per 6 cookies is better than his own sacrifise ratio i.e 1pie : 7 cookies. Bill getting 1 cookie per 0.16 (1/6) pie is better than his own sacrifise ratio i.e 1cookie : 0.25pie (1/5)