Use the market graph before the firm graph
A price-taking firm cannot choose the price delivered by the market.
A competitive firm’s price is $18. Marginal cost for quantities 1 through 5 is $6, $10, $14, $19, and $25. What quantity maximizes profit?
- 3
- 2
- 4
- 5
- 1
3 The firm produces the first three units because their marginal costs do not exceed $18. The fourth unit costs $19 at the margin.
A price-taking bakery receives $1,700 in revenue and incurs $1,400 in variable cost. Its producer surplus is
- $170
- $700
- $300
- $1,400
- $1,700
$300 Producer surplus is revenue minus variable cost: $1,700 – $1,400 = $300. Fixed cost is not subtracted from producer surplus.
A competitive wheat market sets a price of $6 per bushel. What should be carried from the market graph to the graph of one price-taking farm?
- The entire market demand curve
- The market quantity
- A horizontal price line at $6
- The market supply curve
- The number of farms
A horizontal price line at $6 The market intersection determines price. One small farm then treats that price as given, so its demand and marginal-revenue line is horizontal at $6. The farm’s output is found separately where that line meets its marginal cost.
Watch the idea in action
A focused video lesson from Jacob Clifford.
Suppose market demand rises. The market graph first shows a higher short-run equilibrium price. Only then does the representative firm’s horizontal P=MR line move upward. The firm expands output along its unchanged MC curve and may earn economic profit. Drawing only the firm graph can show the response, but it cannot explain where the new price came from.
Use capital Q for industry quantity and lowercase q for one firm’s output. If 200 identical firms each produce 15 units, Q=3,000 while q=15. The market intersection determines the price associated with 3,000. Each firm then chooses 15 at that price. Assigning 3,000 to one firm destroys the price-taking assumption.
| Step | Market graph | Firm graph |
|---|---|---|
| 1 | Shift demand or supply | Hold curves until price is known |
| 2 | Read new equilibrium price and Q |
Transfer price as horizontal P=MR |
| 3 | – | Find q where price meets rising MC |
| 4 | – | Compare price with AVC and ATC |
The reverse sequence follows a market-supply increase. Market price falls. The firm’s revenue line falls. The firm contracts output. If price remains at least AVC, it operates even when price is below ATC. If price falls below minimum AVC, no positive output covers avoidable cost, so shutdown is optimal. Keep market quantity Q and firm quantity q distinct so the industry’s sales are never assigned to one tiny producer.
Demand increase through both graphs
Market demand raises price from $12 to $17. At $12, one firm’s MC intersection is 40 units. At $17 it is 55. ATC at 55 is $14, so the firm earns (17-14)55=$165. The market event sets the new price, the firm graph sets 55, and the ATC gap measures profit.
In the long run, that profit can attract entry, shifting market supply right and changing price again. The short-run firm response and long-run market response are sequential. A question asking “immediately” stops after existing firms expand. One asking “after entry” continues.
A shift in one firm’s MC has negligible effect on market price under the small-firm assumption, though it changes that firm’s output. A shift affecting every firm’s cost shifts market supply and price. The scope of the event determines whether one or both graphs move.
Do not shift the firm’s horizontal demand line right or left. Its vertical position changes when market price changes. Quantity changes occur along MC at the new line. Precise graph language helps keep axes and levels aligned.
From market price to firm profit
The market sets price at $18. One firm’s rising MC equals $18 at 40 units, where AVC is $11 and ATC is $15. The firm produces 40 units and earns (18-15)(40)=$120. The $11 AVC matters for shutdown, not for calculating economic profit.
Transfer one number between graphs
Find industry equilibrium price, carry that price to the firm graph, and then locate the MC intersection. Do not carry the industry’s total quantity to one firm.
The two panels communicate through price while retaining different horizontal quantities.
Reza is an experienced Math instructor and a test-prep expert who has been tutoring students since 2008. He has helped many students raise their standardized test scores--and attend the colleges of their dreams. He works with students individually and in group settings, he tutors both live and online Math courses and the Math portion of standardized tests. He provides an individualized custom learning plan and the personalized attention that makes a difference in how students view math.
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