Suppose a cell phone manufacturer currently sells 20,000 cell phones per week and makes a profit of $5,000 per week. A manager at the plant observes, "Although the last 3,000 cell phones we produced and sold increased our revenue by $6,000 and our costs by $6,700, we are still making an overall profit of $5,000 per week so I think we're on the right track. We are producing the optimal number of cell phones."