These are capability horizons, not calendar lengths. A food cart's "long run" may be weeks; a power plant's may be a decade.
Suppose you run a small bakery with one oven and you add workers one at a time:
| Workers | Total product (loaves/day) | Marginal product (MP) | Average product (AP) |
|---|---|---|---|
| 1 | 30 | 30 | 30 |
| 2 | 70 | 40 | 35 |
| 3 | 100 | 30 | 33.3 |
| 4 | 115 | 15 | 28.75 |
| 5 | 120 | 5 | 24 |
Law of diminishing marginal returns: as more of a variable input (labor) is added to a fixed input (the oven), MP eventually falls — here, beginning with worker 3. Early specialization can make MP rise at first (worker 2: one preps dough while one bakes), but crowding around the fixed oven always wins eventually. Critically, diminishing marginal returns does not mean output falls — total product keeps rising as long as MP is positive; it just rises more slowly.
This is a short-run law: it requires a fixed input.
Per-unit costs:
| Measure | Formula | Behavior |
|---|---|---|
| AFC | TFC ÷ Q | Always falling as Q rises ("spreading the overhead") |
| AVC | TVC ÷ Q | U-shaped |
| ATC | TC ÷ Q = AFC + AVC | U-shaped, above AVC |
| MC | ΔTC ÷ ΔQ = ΔTVC ÷ ΔQ | Falls briefly, then rises (fixed cost never affects MC) |
[GRAPH: Cost curves. X-axis "Quantity", Y-axis "Cost per unit ($)". MC is J-shaped: briefly falling, then rising steeply. AVC and ATC are U-shaped, ATC above AVC, with the vertical gap between them (= AFC) shrinking as Q grows. MC passes through the minimum point of AVC first, then the minimum point of ATC. AFC drawn separately, continuously declining.]
Three relationships the CLEP exam tests constantly:
The official CLEP sample questions include a total-cost table, so make these anchors automatic:
Worked example — a bakery with TFC = $200/day:
| Q (batches) | TC | MC | AVC | ATC |
|---|---|---|---|---|
| 0 | 200 | — | — | — |
| 1 | 260 | 60 | 60 | 260 |
| 2 | 300 | 40 | 50 | 150 |
| 3 | 360 | 60 | 53.3 | 120 |
| 4 | 460 | 100 | 65 | 115 |
| 5 | 610 | 150 | 82 | 122 |
Check the average-marginal rule in the numbers: ATC falls while MC is below it (through Q = 4) and turns up once MC ($150) exceeds ATC.
A bakery locked into a $2,000/month lease is deciding whether to bake one more batch (ingredients + labor $40, expected revenue $55). The lease is unchanged either way — it is fixed and irrelevant to the marginal decision. MR ($55) > MC ($40), so bake it. "Fixed costs don't affect marginal decisions" returns as the shutdown rule in Lesson 9.
Q1 — C. The short run is defined by the presence of at least one fixed input. A describes the long run. B: entry/exit is a long-run capability, not a short-run one. D: the short run is about input flexibility, not calendar time. E: output can change in the short run — by varying labor and materials. Fix: Ask "is anything fixed?" — if yes, short run; if everything can change, long run.
Q2 — D. MP by worker: 8, 12, 16, 8, 4. MP peaks at the 3rd worker (16) and first falls with the 4th (8 < 16). A/B: MP is still rising there — specialization gains. C: the 3rd worker has the highest MP; diminishing returns begins one worker later. E: returns are already diminishing before the 5th. Fix: Compute every MP first; diminishing returns starts at the first worker whose MP is lower than the previous worker's.
Q3 — A. MC = ΔTC/ΔQ (equivalently ΔTVC/ΔQ, since fixed cost doesn't change). B is ATC. C is AVC. D: TFC never changes, so ΔTFC/ΔQ is zero — fixed cost never drives MC. E equals TFC, not MC. Fix: Marginal = change in a total ÷ change in quantity; average = total ÷ quantity.
Q4 — B. AFC = TFC/Q falls forever as a constant fixed cost is spread over more units. A: MC falls briefly, then rises. C and D: AVC and ATC are U-shaped — they fall, bottom out, and rise. E: total fixed cost is constant, not declining — confusing flat TFC with falling AFC is the classic mix-up. Fix: "Spreading the overhead": the only curve that never turns up is AFC.
Q5 — E. AFC = 100/20 = $5, so AVC = ATC − AFC = 15 − 5 = $10. A is AFC itself. B is ATC — forgetting to subtract anything. C multiplies instead of dividing somewhere. D halves ATC with no economic basis. Fix: ATC = AFC + AVC — given any two, subtract to get the third.
Q6 — D. When the marginal is above the average, it pulls the average up, so AVC is rising. A: AVC falls only when MC is below it. B: AVC is at its minimum exactly where MC crosses it, not wherever MC is above it. C: MC vs. AVC says nothing about ATC equality. E: AVC and AFC are unrelated components of ATC. Fix: Marginal below average → average falls; marginal above average → average rises (GPA logic).
Q7 — D. TVC(3) = 20 + 15 + 25 = $60; TC = TFC + TVC = 100 + 60 = $160. A forgets fixed cost entirely. B forgets variable cost. C stacks only the first two MCs (100 + 20 + 15) — stopping one unit short. E stacks all four MCs (through unit 4) instead of three (100 + 100 = $200). Fix: MCs stack into TVC up to exactly Q units; then add TFC once.
Q8 — B. Diminishing marginal returns means each additional worker adds less than the one before — total product still rises as long as MP is positive. A states the misconception itself: falling MP ≠ falling output. C: negative MP (output actually falling) is a further stage, not the definition. D: the law applies to any variable input added to a fixed one, labor included. E: diminishing returns typically appears precisely while hiring continues. Fix: Diminishing returns = the growth of output slows, not output shrinks.
At an output of 10 units, a firm's ATC is $12 and its AVC is $9. The marginal cost of the 11th unit is $10. If the firm produces the 11th unit, its ATC and AVC will change in which of the following ways?
| ATC | AVC | |
|---|---|---|
Q9 — E. MC of the 11th unit ($10) is below ATC ($12) → ATC is pulled down. The same $10 is above AVC ($9) → AVC is pulled up. A assumes any new unit raises both averages. B assumes any MC below ATC lowers everything. C reverses both comparisons. D wrongly treats ATC as unaffected by a unit cheaper than the average. Fix: Compare MC to each average separately — one number can pull ATC down while pulling AVC up.
Q10 — A. MC = wage ÷ MP: rising MP means each extra unit needs less labor time, so MC falls. B: MC rises only once MP diminishes. C: constant MC would require constant MP. D: MC equals AVC only at AVC's minimum point. E: AFC is driven by fixed cost, not productivity. Fix: MP and MC are mirror images — when one rises, the other falls.
Q11 — C. TVC(4) = TC(4) − TFC = 460 − 200 = $260; AVC = 260/4 = $65. A is AFC (200/4) — subtracting the wrong piece. B is the MC of the first batch, confusing marginal with average. D is the MC of the 4th batch (460 − 360). E is ATC (460/4) — forgetting to remove fixed cost. Fix: In any total-cost table, TC at Q = 0 is TFC; strip it out before computing anything "variable."
Q12 — E. The lease is fixed — identical whether or not the batch is baked — so only marginal figures matter: MR ($55) > MC ($40) means the batch adds $15 toward covering the lease. A demands full-cost recovery before marginal decisions, the exact error. B wrongly loads fixed cost into MC. C compares the wrong magnitudes — every profitable batch helps regardless of size. D: current overall profitability is irrelevant; the batch improves the bottom line either way. Fix: For "one more unit?" decisions, compare MR to MC only — fixed costs are already sunk in the short run.
Q1 — C. The short run is defined by the presence of at least one fixed input. A describes the long run. B: entry/exit is a long-run capability, not a short-run one. D: the short run is about input flexibility, not calendar time. E: output can change in the short run — by varying labor and materials. Fix: Ask "is anything fixed?" — if yes, short run; if everything can change, long run.
Q2 — D. MP by worker: 8, 12, 16, 8, 4. MP peaks at the 3rd worker (16) and first falls with the 4th (8 < 16). A/B: MP is still rising there — specialization gains. C: the 3rd worker has the highest MP; diminishing returns begins one worker later. E: returns are already diminishing before the 5th. Fix: Compute every MP first; diminishing returns starts at the first worker whose MP is lower than the previous worker's.
Q3 — A. MC = ΔTC/ΔQ (equivalently ΔTVC/ΔQ, since fixed cost doesn't change). B is ATC. C is AVC. D: TFC never changes, so ΔTFC/ΔQ is zero — fixed cost never drives MC. E equals TFC, not MC. Fix: Marginal = change in a total ÷ change in quantity; average = total ÷ quantity.
Q4 — B. AFC = TFC/Q falls forever as a constant fixed cost is spread over more units. A: MC falls briefly, then rises. C and D: AVC and ATC are U-shaped — they fall, bottom out, and rise. E: total fixed cost is constant, not declining — confusing flat TFC with falling AFC is the classic mix-up. Fix: "Spreading the overhead": the only curve that never turns up is AFC.
Q5 — E. AFC = 100/20 = $5, so AVC = ATC − AFC = 15 − 5 = $10. A is AFC itself. B is ATC — forgetting to subtract anything. C multiplies instead of dividing somewhere. D halves ATC with no economic basis. Fix: ATC = AFC + AVC — given any two, subtract to get the third.
Q6 — D. When the marginal is above the average, it pulls the average up, so AVC is rising. A: AVC falls only when MC is below it. B: AVC is at its minimum exactly where MC crosses it, not wherever MC is above it. C: MC vs. AVC says nothing about ATC equality. E: AVC and AFC are unrelated components of ATC. Fix: Marginal below average → average falls; marginal above average → average rises (GPA logic).
Q7 — D. TVC(3) = 20 + 15 + 25 = $60; TC = TFC + TVC = 100 + 60 = $160. A forgets fixed cost entirely. B forgets variable cost. C stacks only the first two MCs (100 + 20 + 15) — stopping one unit short. E stacks all four MCs (through unit 4) instead of three (100 + 100 = $200). Fix: MCs stack into TVC up to exactly Q units; then add TFC once.
Q8 — B. Diminishing marginal returns means each additional worker adds less than the one before — total product still rises as long as MP is positive. A states the misconception itself: falling MP ≠ falling output. C: negative MP (output actually falling) is a further stage, not the definition. D: the law applies to any variable input added to a fixed one, labor included. E: diminishing returns typically appears precisely while hiring continues. Fix: Diminishing returns = the growth of output slows, not output shrinks.
Q9 — E. MC of the 11th unit ($10) is below ATC ($12) → ATC is pulled down. The same $10 is above AVC ($9) → AVC is pulled up. A assumes any new unit raises both averages. B assumes any MC below ATC lowers everything. C reverses both comparisons. D wrongly treats ATC as unaffected by a unit cheaper than the average. Fix: Compare MC to each average separately — one number can pull ATC down while pulling AVC up.
Q10 — A. MC = wage ÷ MP: rising MP means each extra unit needs less labor time, so MC falls. B: MC rises only once MP diminishes. C: constant MC would require constant MP. D: MC equals AVC only at AVC's minimum point. E: AFC is driven by fixed cost, not productivity. Fix: MP and MC are mirror images — when one rises, the other falls.
Q11 — C. TVC(4) = TC(4) − TFC = 460 − 200 = $260; AVC = 260/4 = $65. A is AFC (200/4) — subtracting the wrong piece. B is the MC of the first batch, confusing marginal with average. D is the MC of the 4th batch (460 − 360). E is ATC (460/4) — forgetting to remove fixed cost. Fix: In any total-cost table, TC at Q = 0 is TFC; strip it out before computing anything "variable."
Q12 — E. The lease is fixed — identical whether or not the batch is baked — so only marginal figures matter: MR ($55) > MC ($40) means the batch adds $15 toward covering the lease. A demands full-cost recovery before marginal decisions, the exact error. B wrongly loads fixed cost into MC. C compares the wrong magnitudes — every profitable batch helps regardless of size. D: current overall profitability is irrelevant; the batch improves the bottom line either way. Fix: For "one more unit?" decisions, compare MR to MC only — fixed costs are already sunk in the short run.