Suppose Carlos is currently using combination D$,$ producing one truck per day. His opportunity cost of producing a second truck per day is per day.

Now, suppose Carlos is currently using combination C$,$ producing two trucks per day. His opportunity cost of producing a third truck per day is per day.

From the previous analysis, you can determine that as Carlos increases his production of trucks, his opportunity cost of producing one more truck

Suppose Carlos buys a new tool that allows him to produce twice as many trucks per hour as before, but it doesn't affect his ability to produce puzzles.

Use the green points $($triangle symbol$)$ to plot his new PPF on the previous graph.

Because Carlos can now make more trucks per hour, Carlos's opportunity cost of producing puzzles is it was previously.