If this society uses all its resources providing housing, it will be able to produce 150 units. If it chooses to produce some food, it can move to point A. It only needs to give up 10 units of housing, reducing housing output from 150 to 140, while food production increases from 0 to 40 units. The opportunity cost of food in terms of housing is very low when all the resources are used for housing production, because some people who are good cooks but lousy builders are nevertheless building, and some land that is ideal for corn or wheat but poor for lumber is still being used to grow trees. If it decides to produce food, the society will first move those resources most suited to food production from housing production, so the loss in housing output will be small but the increase in food output will be large.
18 It's hard to drive a nail with a cookie sheet, and while it may be fun to cut a cake with a chain saw, it's not a good idea.