Questions: Average and Marginal Cost Curves: Shapes and Relationships

Question 1

A firm's marginal cost is currently below its average total cost. As the firm produces one more unit, what happens to its average total cost?

• A. ATC increases — any increase in marginal cost must pull the average upward
• B. ATC decreases — producing a unit cheaper than the current average pulls that average down
• C. ATC stays constant — marginal and average cost only meet at one point and don't affect each other
• D. ATC increases — the firm is past its efficient scale and costs are rising

Answer: (B)

When the marginal cost of a new unit is below the current average, that unit is cheaper than average and pulls the average down — just like scoring below your current exam average lowers your overall average. ATC falls whenever MC < ATC. ATC only rises when MC > ATC. The common mistake in option A is confusing 'marginal cost is positive' with 'marginal cost is rising' — a positive but below-average MC still reduces ATC.

Question 2

A firm's Marginal Cost curve intersects its Average Variable Cost curve and its Average Total Cost curve. At which output levels do these intersections occur?

• A. MC intersects ATC at its minimum, then intersects AVC at its minimum at a higher output level
• B. MC intersects both AVC and ATC at the same output level — the minimum of ATC
• C. MC intersects AVC at its minimum first (lower output), then intersects ATC at its minimum (higher output)
• D. MC intersects neither curve at a minimum — it crosses them at random points depending on the firm's technology

Answer: (C)

MC crosses AVC at AVC's minimum first, then crosses ATC at ATC's minimum at a higher output. AVC reaches its minimum earlier because ATC also benefits from fixed-cost spreading — the declining AFC keeps ATC falling longer before diminishing returns dominate. When MC finally exceeds ATC, it has already been above AVC for some interval. This is why option A reverses the order: the AVC minimum always comes before the ATC minimum.

Question 3

The vertical gap between the ATC curve and the AVC curve at any output level equals the average fixed cost at that output, and this gap narrows as output increases.

• T. True
• F. False

Answer: True

ATC = AVC + AFC, so ATC − AVC = AFC = FC/Q. Since fixed costs FC are constant, AFC declines continuously as Q increases, shrinking the gap between ATC and AVC toward zero. This is why the two curves converge as output rises but never intersect — AFC is always positive (fixed costs are never zero), so ATC always lies above AVC.

Question 4

The minimum point of ATC and the minimum point of AVC occur at the same output level, since both curves are U-shaped for the same underlying reason.

• T. True
• F. False

Answer: False

They are U-shaped for partially the same reason (diminishing returns), but ATC also benefits from fixed-cost spreading, which keeps ATC declining for longer. AVC's minimum is reached at a lower output level than ATC's minimum. Marginal cost crosses AVC at AVC's minimum, then continues rising until it crosses ATC at ATC's minimum. The two minima occur at different output levels, and confusing them leads to errors in analyzing shut-down decisions versus exit decisions.

Question 5

Why does the marginal cost curve always intersect both the AVC and ATC curves at their respective minimum points? Explain using the logic of how marginal values affect averages.

Model answer: When a marginal value (cost of the next unit) is below the current average, it pulls the average down; when it is above the average, it pulls the average up; they are equal precisely at the average's minimum. Below the ATC minimum, MC < ATC, so each new unit is cheaper than average and ATC falls. Above the ATC minimum, MC > ATC, so each new unit is more expensive than average and ATC rises. The crossing point — where MC = ATC — is exactly the turning point of the ATC curve. The same logic applies to AVC. This is the 'marginals pull averages' principle.

This principle extends beyond cost curves: it applies whenever you compare marginal and average quantities. If a new data point is above the current average, the average rises; if below, it falls; equality defines the extremum. In production theory, this connects to diminishing returns — MC rises due to diminishing marginal product of variable inputs, and once it exceeds the average, the average inevitably rises too.