Suppose initially that the United States is consuming 22 boots and 1818 shirts and Canada is consuming 1212 boots and 22 shirts

If the US is consuming 22 boots and 1818 shirts, we can standardize this to find that the US is consuming 1 boot per 82.6 shirts. This is the USA's opportunity cost of boots. If Canada is consuming 1212 boots and 22 shirts, we can standardize this to find that Canada is consuming 1 boot per 0.018 shirts. This is Canada's opportunity cost of boots. Since Canada has the lower opportunity cost of boots, it has a comparative advantage in making boots and should specialize in boot production.

The US should specialize in the other good: shirts. In order to do this, it will give up 22 boots and gain 82.6 * 22 = 1833 shirts. Thus, the US will end up with 1818 + 1833 = 3651 shirts after specializing. If it sends half of its production to Canada for boots, it will have 1826 shirts left over. It started with 1818 shirts, so this is a net gain of 1826 - 1818 = 8 shirts after trading with Canada.