Result: each firm is a price taker. Think of a commodity crop farmer or a gig-economy driver on a platform with posted rates: the market (or the app) sets the price; the individual's only real decision is how much to produce.
Market demand slopes downward as always, but the individual firm faces a horizontal (perfectly elastic) demand curve at the market price:
P = MR = AR = D(firm)
Why MR = P: each extra unit sells at the going market price without lowering the price received on earlier units. (In later lessons, firms with downward-sloping demand will have MR < P — this contrast is a favorite exam question.)
Produce every unit for which MR ≥ MC; stop where MR = MC (with MC rising). Producing less leaves profitable units on the table; producing more adds units that cost more than they bring in. Since MR = P here, the rule becomes P = MC.
MR = MC picks the quantity only. Whether that quantity yields profit or loss is a separate step:
Economic profit = (P − ATC) × Q
[GRAPH: Two panels. LEFT (Market): downward-sloping D and upward-sloping S crossing at market price P and market quantity Q. RIGHT (Firm): horizontal line at P labeled MR = D = AR = P; J-shaped MC crossing it at q*; U-shaped ATC and AVC below. The price line carries across from the left panel to the right at the same height.]
| Case | Condition at q* | Action | Result |
|---|---|---|---|
| Economic profit | P > ATC | Produce where MR = MC | Profit rectangle: (P − ATC) × q* |
| Loss, keep operating | AVC < P < ATC | Produce where MR = MC | Loss < TFC; revenue covers all variable costs plus part of fixed costs |
| Shutdown | P < minimum AVC | Produce zero | Loss = TFC |
P ≥ minimum AVC → produce where MR = MC
P < minimum AVC → shut down (Q = 0, lose TFC)
Fixed costs are irrelevant to the operate/shutdown choice — they are paid either way in the short run (Lesson 7's lease logic). Consequently, the firm's short-run supply curve is its MC curve above minimum AVC. The long-run exit rule is stricter: leave if P stays below ATC, because in the long run all costs must be covered.
Totals-based problems convert to per-unit in one division: P = TR/Q, AVC = TVC/Q, ATC = TC/Q. Do that first.
Free entry and exit relentlessly hunts down economic profit:
Long-run equilibrium: P = MR = MC = minimum ATC. Every firm earns exactly normal profit (Lesson 8's zero that isn't sad), and two efficiency benchmarks are met:
Perfect competition is the only market structure that delivers both efficiencies in the long run; every other structure is judged against it.
Narrate any demand-shock question in three beats: (1) market moves (D shifts, new P); (2) firm reacts (MR line jumps, slide along MC to new q*); (3) entry or exit resets price to minimum ATC and profit to zero.
Q1 — D. Perfect competition requires many sellers of identical products (plus free entry/exit and full information). A describes monopolistic competition. B describes monopoly or oligopoly conditions. C: competitive firms are price takers — they choose quantity, not price. E: the market demand slopes down; each firm's demand is horizontal. Fix: Perfect competition checklist — many, identical, free entry, full information; if any item fails, it's a different structure.
Q2 — C. A price taker can sell any quantity at the market price but nothing above it — a horizontal, perfectly elastic demand curve at P. A confuses the firm's demand with the market's. B: perfectly inelastic would mean buyers take a fixed quantity at any price — backwards. D: unit elasticity describes a specific revenue property, not price taking. E: an upward-sloping curve above AVC is the firm's supply curve, not its demand. Fix: Market panel demand slopes down; firm panel demand is a flat line at the market price.
Q3 — E. Each additional unit sells at the unchanged market price, so MR = P. A describes firms with downward-sloping demand (later lessons) — the single most common cross-structure confusion. B: MR can never exceed the price received. C: in long-run equilibrium MR equals price, which is positive. D: MR relates to revenue; ATC to cost — they coincide only by accident. Fix: Horizontal demand → MR = P; downward-sloping demand → MR < P.
Q4 — E. Profit = (P − ATC) × Q = (10 − 7) × 400 = $1,200. A is only the per-unit margin — forgot to multiply by output. B uses AVC instead of ATC, ignoring fixed costs: (10 − 6) × 400. C is total cost (7 × 400). D is total revenue (10 × 400) with costs ignored. Fix: Profit is a rectangle: height (P − ATC), width Q — compute both dimensions, then multiply.
Q5 — B. P ($5) exceeds minimum AVC ($4.50), so every unit sold covers its variable cost and contributes $0.50 toward fixed costs; operating loses less than shutting down. A applies the wrong threshold — P below ATC signals a loss, not a shutdown. C: a price taker cannot raise price; buyers vanish. D: exit is a long-run decision; in the short run the firm can only operate or produce zero. E: a firm cannot slide its ATC down to the price by producing more — beyond the MR = MC output, extra units make things worse. Fix: Shutdown compares P to minimum AVC, never to ATC.
Q6 — A. The firm produces whenever P exceeds minimum AVC ($6); its supply curve is the MC branch above that point. B confuses the shutdown threshold with the break-even point (minimum ATC) — between $6 and $9 the firm operates at a loss on purpose. C splits the difference with no economic meaning. D ignores the shutdown rule entirely. E reverses the two horizons — the long-run threshold is the higher one ($9), the short-run threshold the lower ($6). Fix: Short run: produce if P > min AVC; long run: stay only if P ≥ min ATC.
Q7 — C. Economic profits attract entrants; entry shifts market supply right; price falls; the process stops only when economic profit reaches zero. A: price takers cannot raise price. B describes the response to losses, reversed. D: with free entry, incumbents have no price control and profits cannot be defended. E: demand has no automatic reason to change; supply-side entry does the adjusting. Fix: Profits → entry → supply right → price down to minimum ATC — always tell the story to its zero-profit endpoint.
A perfectly competitive industry is suffering short-run losses. As the industry adjusts to long-run equilibrium, market supply and market price will change in which of the following ways?
| Market Supply | Market Price | |
|---|---|---|
Q8 — B. Losses drive firms out; market supply decreases (shifts left); with less supply, market price rises until survivors again break even at minimum ATC. A is the profit-case adjustment, reversed. C: a supply decrease raises price along the demand curve — supply and price cannot both fall here. D: entry (supply increase) is triggered by profits, not losses. E: supply must change — exit is the entire adjustment mechanism. Fix: Losses → exit → supply left → price up; profits → entry → supply right → price down.
Q9 — C. TR ($900) exceeds TVC ($600), so operating covers all variable costs and $300 of the $400 fixed cost: operating loss = $100 versus $400 if it shuts down. A uses the wrong test — TC > TR shows a loss, not a shutdown signal. B: covering fixed cost is not required in the short run; partially covering it is exactly why the firm stays open. D: the relevant comparison is revenue vs. variable cost, not fixed cost. E: TC = $1,000 > TR = $900, so the firm is running a $100 loss, not a profit. Fix: Totals version of the shutdown rule: operate if TR > TVC; the excess chips away at fixed costs.
Q10 — B. Zero long-run economic profit is a feature, not a failure: it coexists with production at minimum ATC (productive efficiency), P = MC (allocative efficiency), and owners earning normal profit — covering all opportunity costs. A confuses economic profit with cost coverage; zero economic profit means all costs, including implicit ones, are covered. C: zero economic profit implies positive accounting profit (equal to implicit costs). D is factually wrong — entry eliminates long-run economic profit. E: fixed costs don't bear on the efficiency claims at all. Fix: Evaluate market structures on efficiency (min ATC and P = MC), not on whether firms keep economic profit.
Q11 — D. Higher demand raises the market price; the firm's horizontal MR line jumps up, so it slides up its MC curve to a larger q, and with P now above ATC it earns short-run economic profit. A: price takers don't set prices, and the market price rose. B ignores that the firm's MR changed even though its costs didn't. C: zero profit returns only in the long run, after entry — not in the short run. E: the firm should move MC up to the new price, not hold it at the old one. Fix:* Demand shock sequence — market price moves first, firm re-solves MR = MC at the new price, entry/exit cleans up profits later.
Q12 — A. The $800 lease is fixed this month — lost whether or not she drives. Driving covers all variable costs plus $300 of the lease: loss = $500 driving versus $800 idle. Her revenue exceeds variable cost, so she should keep driving (P > AVC). B applies "loss → quit" without comparing the loss to fixed costs. C demands full-cost coverage for a short-run decision — the Lesson 7 fixed-cost error. D mislabels the lease: it's unavoidable this month, hence fixed. E: a gig driver is a price taker; posting rates above the platform's market rate just means no jobs. Fix: When losing money, compare the operating loss to total fixed cost — operate as long as revenue beats variable cost.
Q1 — D. Perfect competition requires many sellers of identical products (plus free entry/exit and full information). A describes monopolistic competition. B describes monopoly or oligopoly conditions. C: competitive firms are price takers — they choose quantity, not price. E: the market demand slopes down; each firm's demand is horizontal. Fix: Perfect competition checklist — many, identical, free entry, full information; if any item fails, it's a different structure.
Q2 — C. A price taker can sell any quantity at the market price but nothing above it — a horizontal, perfectly elastic demand curve at P. A confuses the firm's demand with the market's. B: perfectly inelastic would mean buyers take a fixed quantity at any price — backwards. D: unit elasticity describes a specific revenue property, not price taking. E: an upward-sloping curve above AVC is the firm's supply curve, not its demand. Fix: Market panel demand slopes down; firm panel demand is a flat line at the market price.
Q3 — E. Each additional unit sells at the unchanged market price, so MR = P. A describes firms with downward-sloping demand (later lessons) — the single most common cross-structure confusion. B: MR can never exceed the price received. C: in long-run equilibrium MR equals price, which is positive. D: MR relates to revenue; ATC to cost — they coincide only by accident. Fix: Horizontal demand → MR = P; downward-sloping demand → MR < P.
Q4 — E. Profit = (P − ATC) × Q = (10 − 7) × 400 = $1,200. A is only the per-unit margin — forgot to multiply by output. B uses AVC instead of ATC, ignoring fixed costs: (10 − 6) × 400. C is total cost (7 × 400). D is total revenue (10 × 400) with costs ignored. Fix: Profit is a rectangle: height (P − ATC), width Q — compute both dimensions, then multiply.
Q5 — B. P ($5) exceeds minimum AVC ($4.50), so every unit sold covers its variable cost and contributes $0.50 toward fixed costs; operating loses less than shutting down. A applies the wrong threshold — P below ATC signals a loss, not a shutdown. C: a price taker cannot raise price; buyers vanish. D: exit is a long-run decision; in the short run the firm can only operate or produce zero. E: a firm cannot slide its ATC down to the price by producing more — beyond the MR = MC output, extra units make things worse. Fix: Shutdown compares P to minimum AVC, never to ATC.
Q6 — A. The firm produces whenever P exceeds minimum AVC ($6); its supply curve is the MC branch above that point. B confuses the shutdown threshold with the break-even point (minimum ATC) — between $6 and $9 the firm operates at a loss on purpose. C splits the difference with no economic meaning. D ignores the shutdown rule entirely. E reverses the two horizons — the long-run threshold is the higher one ($9), the short-run threshold the lower ($6). Fix: Short run: produce if P > min AVC; long run: stay only if P ≥ min ATC.
Q7 — C. Economic profits attract entrants; entry shifts market supply right; price falls; the process stops only when economic profit reaches zero. A: price takers cannot raise price. B describes the response to losses, reversed. D: with free entry, incumbents have no price control and profits cannot be defended. E: demand has no automatic reason to change; supply-side entry does the adjusting. Fix: Profits → entry → supply right → price down to minimum ATC — always tell the story to its zero-profit endpoint.
Q8 — B. Losses drive firms out; market supply decreases (shifts left); with less supply, market price rises until survivors again break even at minimum ATC. A is the profit-case adjustment, reversed. C: a supply decrease raises price along the demand curve — supply and price cannot both fall here. D: entry (supply increase) is triggered by profits, not losses. E: supply must change — exit is the entire adjustment mechanism. Fix: Losses → exit → supply left → price up; profits → entry → supply right → price down.
Q9 — C. TR ($900) exceeds TVC ($600), so operating covers all variable costs and $300 of the $400 fixed cost: operating loss = $100 versus $400 if it shuts down. A uses the wrong test — TC > TR shows a loss, not a shutdown signal. B: covering fixed cost is not required in the short run; partially covering it is exactly why the firm stays open. D: the relevant comparison is revenue vs. variable cost, not fixed cost. E: TC = $1,000 > TR = $900, so the firm is running a $100 loss, not a profit. Fix: Totals version of the shutdown rule: operate if TR > TVC; the excess chips away at fixed costs.
Q10 — B. Zero long-run economic profit is a feature, not a failure: it coexists with production at minimum ATC (productive efficiency), P = MC (allocative efficiency), and owners earning normal profit — covering all opportunity costs. A confuses economic profit with cost coverage; zero economic profit means all costs, including implicit ones, are covered. C: zero economic profit implies positive accounting profit (equal to implicit costs). D is factually wrong — entry eliminates long-run economic profit. E: fixed costs don't bear on the efficiency claims at all. Fix: Evaluate market structures on efficiency (min ATC and P = MC), not on whether firms keep economic profit.
Q11 — D. Higher demand raises the market price; the firm's horizontal MR line jumps up, so it slides up its MC curve to a larger q, and with P now above ATC it earns short-run economic profit. A: price takers don't set prices, and the market price rose. B ignores that the firm's MR changed even though its costs didn't. C: zero profit returns only in the long run, after entry — not in the short run. E: the firm should move MC up to the new price, not hold it at the old one. Fix:* Demand shock sequence — market price moves first, firm re-solves MR = MC at the new price, entry/exit cleans up profits later.
Q12 — A. The $800 lease is fixed this month — lost whether or not she drives. Driving covers all variable costs plus $300 of the lease: loss = $500 driving versus $800 idle. Her revenue exceeds variable cost, so she should keep driving (P > AVC). B applies "loss → quit" without comparing the loss to fixed costs. C demands full-cost coverage for a short-run decision — the Lesson 7 fixed-cost error. D mislabels the lease: it's unavoidable this month, hence fixed. E: a gig driver is a price taker; posting rates above the platform's market rate just means no jobs. Fix: When losing money, compare the operating loss to total fixed cost — operate as long as revenue beats variable cost.