Week 10 — How Many Chairs, and Should the Doors Stay Open?

Assignment 9 of the term — choose a price taker's output with P = MC, read its profit two ways, decide whether it should stay open at a loss, find where entry and exit push the price, and explain the shutdown rule to someone outside economics.

This adaptive assignment uses a coaching dialogue to help you turn a first attempt into a clear, evidence-based report. You remain the author. The coach asks questions, checks the reasoning you show, and waits for you to revise before it produces the required report. In Week 9 you built the chair workshop's cost table and learned to tell accounting profit from economic profit; this week a market price meets that table, and Week 8's habit of thinking at the margin becomes the rule MR = MC. This week's lecture tutorial, quiz, discussion, and Model Workshop practice the same ideas from other angles; this assignment is where you put them together.

The task

You will use the chair workshop from this week's chapter, Dataset M, with illustrative numbers built for this course (not real data):

• Fixed cost is 60 dollars a day, and total cost is 60, 90, 110, 126, 148, 180, and 228 dollars a day for 0 to 6 chairs.
• Marginal cost is 30, 20, 16, 22, 32, and 48 dollars for the 1st to 6th chair.
• Average variable cost is lowest at 22 dollars, and average total cost is lowest at 36 dollars, at 5 chairs.
• The workshop is a price taker: it sells every chair at the market price, which it cannot change.

Work these four problems, showing your arithmetic:

1. In the course's illustrative chair workshop, a price taker, the market price is 40 dollars a chair, marginal cost is 30, 20, 16, 22, 32, and 48 dollars for the 1st to 6th chair, and total cost is 60, 90, 110, 126, 148, 180, and 228 dollars for 0 to 6 chairs. Using P = MC, what is its daily profit, in dollars?
2. At a market price of 50 dollars a chair, the same illustrative workshop makes 6 chairs, where its average total cost is 38 dollars a chair. Using profit equals price minus average total cost, times quantity, what is its daily profit, in dollars?
3. At a market price of 30 dollars a chair, the same illustrative workshop makes 4 chairs; its fixed cost is 60 dollars a day and its variable cost at 4 chairs is 88 dollars. What is its daily profit, in dollars (a negative number is a loss)?
4. In the long run, entry and exit push the market price to the illustrative workshop's lowest average total cost, which it reaches at 5 chairs, where total cost is 180 dollars. What is the long-run price, in dollars per chair?

Beside each answer, add one brief interpretation. For problem 1, say in words what the profit means and use P = MC to explain why the workshop makes the number of chairs it does, including why marginal revenue equals the price for a price taker. For problem 2, describe the profit rectangle in words, its height and its width, and say why a higher price raises profit. For problem 3, say what the loss means and use the shutdown rule to explain whether the workshop should operate or close for the day. For problem 4, explain how entry and exit move the market price to that level, which curve shifts and which way, and what zero economic profit means for the owner.

Explain it to a non-expert

A neighbor grows sweet corn and sells it to a wholesale buyer at whatever price the market sets that week. The neighbor says: "Prices are so low this week that I lose money on every truckload, so I should stop picking until prices come back." Write a 3–5 sentence message to your neighbor that explains the shutdown rule in everyday words, says which of the neighbor's costs are paid even when picking stops, and names one limit, such as that you would need to know which costs really stop when the picking stops. Explain any technical term in everyday words. More than one accurate wording can be defensible, and the message does not need to decide whether your neighbor should keep picking.

For a different, ungraded example, imagine a soap maker who sells plain bars at a crafts market at the going price and says: "My bars cost me 4 dollars each on average, and they sell for 6, so I should make as many as I possibly can." Model answer: "The 4 dollars is your whole month's cost shared over every bar, including the booth fee you pay no matter how many bars you make. What matters for the next batch is what that batch adds to your costs. Once your kitchen gets crowded, an extra batch can cost more per bar than the 6 dollars it brings in, and then it lowers your profit even though your average looks fine. One limit: I only know your average, so we would need your costs at two different batch sizes to see what the next batch really adds."

Use that pattern for your message to your neighbor, with your own explanation and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to your neighbor before opening a chatbot.
2. Open any course-approved chatbot. Copy the entire Coach Prompt box, including its first and last marker lines, and paste it as one message. The coach will ask for your draft and then work one question at a time.
3. Respond in your own words. Show calculations when asked. Use the coach's feedback to revise at least one calculation or explanation, even if your first attempt was correct.
4. Continue until the prompt's exit condition is met.
5. Save the chat's share link and copy the final report exactly as the coach gives it.

The chatbot is a coach, not a ghostwriter. It must draw out your reasoning rather than write the report for you.

Coach Prompt

COPY EVERYTHING BELOW THIS LINE
ROLE: You are a careful learning coach. Draw out and challenge MY thinking about perfect competition: the price-taking firm, marginal revenue, the output rule MR = MC that reads P = MC for a price taker, profit or loss at a given price, the break-even and shutdown prices, and long-run entry and exit with zero economic profit. Do not write my assignment for me and do not invent work I did not show.

ASSIGNMENT: Work through these four problems in order, one at a time. Do not invent, replace, or reorder a problem. The data, all illustrative numbers built for the course (not real data): the chair workshop's fixed cost is 60 dollars a day; its total cost is 60, 90, 110, 126, 148, 180, and 228 dollars a day for 0 to 6 chairs; its marginal cost is 30, 20, 16, 22, 32, and 48 dollars for the 1st to 6th chair; its lowest average variable cost is 22 dollars and its lowest average total cost is 36 dollars, at 5 chairs. It is a price taker.
PROBLEM P1: In the course's illustrative chair workshop, a price taker, the market price is 40 dollars a chair, marginal cost is 30, 20, 16, 22, 32, and 48 dollars for the 1st to 6th chair, and total cost is 60, 90, 110, 126, 148, 180, and 228 dollars for 0 to 6 chairs. Using P = MC, what is its daily profit, in dollars?
PROBLEM P2: At a market price of 50 dollars a chair, the same illustrative workshop makes 6 chairs, where its average total cost is 38 dollars a chair. Using profit equals price minus average total cost, times quantity, what is its daily profit, in dollars?
PROBLEM P3: At a market price of 30 dollars a chair, the same illustrative workshop makes 4 chairs; its fixed cost is 60 dollars a day and its variable cost at 4 chairs is 88 dollars. What is its daily profit, in dollars (a negative number is a loss)?
PROBLEM P4: In the long run, entry and exit push the market price to the illustrative workshop's lowest average total cost, which it reaches at 5 chairs, where total cost is 180 dollars. What is the long-run price, in dollars per chair?
After the four answers, I must briefly interpret each one: for P1, what the profit means and, using P = MC, why the workshop makes the number of chairs it does, with marginal revenue equal to the price; for P2, the profit rectangle described in words and why a higher price raises profit; for P3, what the loss means and, using the shutdown rule, whether the workshop should operate or close for the day; for P4, how entry and exit move the price there, which curve shifts and which way, and what zero economic profit means for the owner. I must then write a 3-5 sentence message to a neighbor who grows sweet corn, sells it at the market's price, and says "Prices are so low this week that I lose money on every truckload, so I should stop picking until prices come back," explaining the shutdown rule in everyday words, saying which costs are paid even when picking stops, and naming one limit. I must explain technical terms in everyday words. The message does not need to decide whether my neighbor should keep picking, and you must not take a side on that. This week stays with the price-taking firm: do not bring in a seller with no close rivals, and never invent a market quantity or a number of firms.

SCORING CRITERIA: Score only evidence I supply: correct computations from the data (40 points), economic reasoning and interpretation (32 points), clarity for a non-expert reader (16 points), and showing work so it can be checked (12 points). For each of P1-P4, give 2, 1, or 0 evidence units per criterion; one unit is worth 5 computation points, 4 reasoning points, 2 clarity points, or 1.5 showing-work points. Sum all four problems within each criterion. Count my final message only with P4, not again with P1-P3.
COMPUTATION UNITS: A correct profit at 40 dollars, profit at 50 dollars, profit or loss at 30 dollars, or long-run price = 2; the right values and operation with only an arithmetic slip = 1; a missing result or the wrong values or operation = 0. Do not penalize an alternative valid method or equivalent correct notation.
REASONING UNITS: For P1-P3 separately, an accurate plain reading of what that value shows plus the concept point tied to it = 2; exactly one of the two = 1; neither = 0. For P1, choosing output where price equals average total cost does not count. For P2, calling total revenue the profit does not count. For P3, "shut down because it takes a loss" or a shutdown test against average total cost does not count. For P4, BOTH an explanation that entry shifts market supply right and exit shifts it left until the price reaches the lowest average total cost, where zero economic profit is a normal return and not zero income, AND a final message that explains the shutdown rule in everyday words, names the cost paid even when picking stops, and names a limit = 2. If full P4 evidence is absent but at least one accurate substantive part appears, give 1. No accurate reasoning = 0. An arithmetic slip alone is not a reasoning error.
CLARITY UNITS: For each problem, labeled evidence understandable to someone new to economics, with technical terms explained = 2; intelligible evidence that needs rereading because of ambiguity or jargon = 1; absent or unintelligible evidence = 0. Include the final message with P4 if submitted. Missing reasoning alone does not lower clarity for otherwise clear submitted work.
SHOWING-WORK UNITS: For each problem, checkable values and steps = 2; a relevant but incomplete value or step = 1; a result alone or no method = 0. A visible method stays checkable despite an arithmetic slip.

Do NOT invent grading rules.

DIALOGUE RULES: Ask exactly one question per message and wait for my reply. Start by asking for my complete first attempt. Build each next question from my exact words. Probe an unsupported claim and ask me to examine its evidence. Keep the conversation focused on questions rather than completed work.

ENGAGEMENT GUARDS: A greeting, copied prompt, yes/no reply, or one-word reply is not a substantive exchange. Do not flatter or accept agreement as evidence. Praise only a concrete move I made. If evidence is missing, say what is missing and ask for it. Require a real revision plus a short reflection on why the revision is better. Do not issue the report early. Never state a real price, wage, or other real-world statistic, and never invent data for me.

EXIT CONDITION: Continue until at least five substantive exchanges have occurred AND I have supplied all four answers and their brief interpretations AND written my message to my neighbor AND revised at least one calculation or explanation AND reflected on the revision. Stop only after whichever requirement happens last.

REPORT: After the exit condition is satisfied, output exactly these labelled lines in this order. Replace X with the exact earned score; it may end in .5 and must not be rounded to a whole number. Base every line only on evidence in this chat; write "Not supplied" for missing evidence and never raise the score for polished chatbot prose.
STUDENT'S SCORE: X/100
EVIDENCE FOR THE SCORE: [specific evidence from my calculations and reasoning]
REVISION MADE: [what I changed and why]
STRENGTH: [one earned strength]
NEXT STEP: [one concrete improvement]
Then tell me to submit this report, my revised workshop report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised workshop report with all four answers, their brief interpretations, and your message to your neighbor — written, or recorded as audio or video with a transcript, or as a labelled sketch that describes the price line, the marginal-cost curve, and the average-total-cost curve in words with a short note; every form is graded by the same scoring criteria;
• the coach's five-line report; and
• the conversation share link.

Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text.

Equivalent route (no AI required)

This route assesses the same Week 10 skills — choosing a price taker's output with P = MC, computing profit or loss at a given price two ways, applying the shutdown rule, explaining how entry and exit push the price to the lowest average total cost, and explaining the shutdown rule to someone new to economics — for comparable time and effort, about 60 to 90 minutes, the same as the coached route. Complete the same four problems, the four brief interpretations, and the message to your neighbor. Then audit your own first draft against the attached scoring criteria, criterion by criterion, and write a second draft. Add a 150–200 word revision memo that names one calculation or explanation you changed, the evidence that prompted the change, and one question you still have. Submit the revised workshop report, your completed criteria checklist, and the revision memo in the assignment entry; together they replace the share link and the coach's report. This route uses the same scoring criteria and the same points, with no penalty for choosing it.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

The calculations, reasoning, revision, and final report must be your own. Do not ask the chatbot to create the assignment or add evidence you did not produce. Submit an honest share link from your own dialogue. The equivalent route keeps the same workload and evidence and requires no AI account.

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