Could you help me with this question? I calculated that the Portugal's opportunity cost of Hairbrush (also its competitive advantage) is 6/5. For the two countries not to gain from trade, the opportunity cost of Zimbabwe's toothbrush must equal that of Portugal, which is 6/5. However the answer to this question is 5/6. Could you please explain? Thank you very much.

Assume that Zimbabwe and Portugal can switch between producing toothbrushes and producing hairbrushes at a constant rate. Machine Minutes Needed to Make 1 Toothbrush Hairbrush Zimbabwe 10 Portugal 5 6 1. Refer to Table 3-22. Zimbabwe and Portugal would not be able to gain from trade if Zimbabwe's opportunity cost of one toothbrush changed to a. 0 hairbrushes. b. 5/6 hairbrushes. c. 6/5 hairbrushes. d. Zimbabwe and Portugal can always gain from trade regardless of their opportunity costs