Suppose Kenji is currently using combination D, producing 1 train per day. His opportunity cost of producing a second train per day is 2 balls per day.
Now, suppose Kenji is currently using combination C, producing 2 trains per day. His opportunity cost of producing a third train per day is 6 balls per day.
From the previous analysis, you can determine that as Kenji increases his production of trains, his opportunity cost of producing 1 more train increases
Suppose Kenji buys a new tool that enables him to produce twice as many trains per hour as before, but it doesn't affect his ability to produce balls.
Use the green points (triangle symbol) to plot his new PPF on the previous graph.
Because he can now make more trains per hour, Kenji's opportunity cost of producing balls is it was previously.
