Modify Solved Problem 10.5 to show that the PPF more closely approximates a quarter of a circle

Question:

Modify Solved Problem 10.5 to show that the PPF more closely approximates a quarter of a circle if there are six people. One of these new people, Bill, can produce five cords of wood, or four candy bars, or any linear combination. The other, Helen, can produce four cords of wood, or five candy bars, or any linear combination?