Solution and Explanation:
1st question:
The optimal quantity could be found where MB = MC
Where, MB = marginal benefit, and MC = Marginal cost
The corresponding point on the horizontal axis would be the optimal quantity.
Answer: 5 cans
2nd question:
The marginal benefit (MB) from additional can is no longer worth the cost. Suppose he consumes 6 numbers of cans. At this level MC > MB, and this is not worthwhile because the cost becomes higher than benefits.
Answer: The exra energy benefits Patrick gets from another can are no longer worth the cost.