Week 3 — Reading the Demand Curve: Slides, Shifts, and Buyers

Assignment 3 of the term — build quantities from this week's illustrative demand schedule, tell a slide along the demand curve from a shift of the whole curve, add buyers into market demand, and explain to someone outside economics why a price cut does not raise demand.

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 1 you learned that economists hold everything else fixed, ceteris paribus, to see one cause at a time. This week the law of demand uses exactly that habit. 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 these materials, all illustrative numbers built for this course (not real data) and all from this week's chapter:

• The original demand rule: Qd = 90 − 10P, so buyers want 50 units a day at 4 dollars, 40 at 5, 30 at 6, 20 at 7, and 10 at 8.
• The shifted rule after incomes rise for a normal good: Qd = 110 − 10P, 20 more units at every price.
• The ten-buyer reading of the same market: each buyer wants q = 9 − P units a day.

Work these four problems, showing your arithmetic:

1. In the illustrative market, demand is Qd = 90 − 10P, with P in dollars and Qd in units per day. What is quantity demanded at a price of 7 dollars?
2. In the same illustrative market, the good's own price falls from 8 dollars to 5 dollars and nothing else changes. By how many units per day does quantity demanded rise?
3. Buyers' incomes rise, the good is a normal good, and demand in the illustrative market becomes Qd = 110 − 10P. What is quantity demanded at a price of 6 dollars on the new curve?
4. Read the illustrative market as ten identical buyers, each wanting q = 9 − P units a day. What is the market's quantity demanded at a price of 5 dollars?

Beside each answer, add one brief interpretation. For problem 1, say in words what the quantity means and tie it to the law of demand. For problem 2, explain whether the change is a movement along the demand curve or a shift of it, and use the words "quantity demanded" or "demand" correctly. For problem 3, name the curve that moved, its direction, and the determinant that moved it, and compare the new quantity with the original 30 units at 6 dollars. For problem 4, explain how market demand adds buyers' quantities at one price, and check your total against the schedule.

Explain it to a non-expert

A food-truck owner has never studied economics. After cutting the price of tacos, the owner sold more and now plans to announce: "My price cut raised demand for my tacos, so the extra sales will stay even after I go back to the old price." Write a 3–5 sentence message to the owner that recommends a more accurate way to say it, explains in everyday words the difference between buyers sliding along the same demand curve and the whole curve moving, says what kind of change would signal a real shift, and names one limit, such as that one week of sales alone cannot show what caused the change. Explain any technical term in everyday words. More than one accurate wording can be defensible, and the message does not need to judge whether the price cut was a good business choice.

For a different, ungraded example, imagine a hardware-store manager who says: "We sold three times as many umbrellas during the storm week, so our umbrella price must have dropped." Model answer: "Our price stayed the same, so the jump did not come from a slide along the demand curve. The storm changed what shoppers wanted, which economists call a change in tastes or conditions, and that moves the whole demand curve to the right: people want more umbrellas at every price. Once the storm passes, that extra demand will likely fade. One limit: one stormy week does not tell us how many umbrellas we would sell at a different price."

Use that pattern for your message to the food-truck owner, with your own recommendation and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to the food-truck owner 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 demand: the law of demand, the demand schedule and curve, market demand as a sum of buyers, and the difference between a movement along the demand curve (a change in quantity demanded) and a shift of the curve (a change in demand). 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 original demand rule Qd = 90 − 10P gives 50 units a day at 4 dollars, 40 at 5, 30 at 6, 20 at 7, and 10 at 8; after incomes rise for a normal good, demand becomes Qd = 110 − 10P, 20 more units at every price; read as ten identical buyers, each buyer wants q = 9 − P units a day.
PROBLEM P1: In the illustrative market, demand is Qd = 90 − 10P, with P in dollars and Qd in units per day. What is quantity demanded at a price of 7 dollars?
PROBLEM P2: In the same illustrative market, the good's own price falls from 8 dollars to 5 dollars and nothing else changes. By how many units per day does quantity demanded rise?
PROBLEM P3: Buyers' incomes rise, the good is a normal good, and demand in the illustrative market becomes Qd = 110 − 10P. What is quantity demanded at a price of 6 dollars on the new curve?
PROBLEM P4: Read the illustrative market as ten identical buyers, each wanting q = 9 − P units a day. What is the market's quantity demanded at a price of 5 dollars?
After the four answers, I must briefly interpret each one: for P1, what the quantity means and how it shows the law of demand; for P2, whether the change is a movement along the curve or a shift, using "quantity demanded" and "demand" correctly; for P3, which curve moved, which way, and which determinant moved it, compared with the original 30 units at 6 dollars; for P4, how market demand adds buyers' quantities at one price, checked against the schedule. I must then write a 3-5 sentence message to a food-truck owner who is new to economics and plans to announce "My price cut raised demand for my tacos, so the extra sales will stay even after I go back to the old price," recommending a more accurate wording, explaining in everyday words a slide along the curve versus a shift of the whole curve, saying what kind of change would signal a real shift, and naming one limit. I must explain technical terms in everyday words. The message does not need to judge whether the price cut was a good business choice, and you must not take a side on that. This week is demand only: do not ask me to find a market price or an equilibrium.

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 quantity demanded, change in quantity demanded, new-curve quantity, or market quantity = 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, the quantity must be tied to the law of demand. For P2, calling the price fall an increase in demand or a shift of the curve does not count. For P3, a shift without its direction and determinant does not count. For P4, BOTH an explanation of adding quantities at one price checked against the schedule AND a final message that recommends an accurate wording, explains a slide along the curve versus a shift, says what would signal a real shift, 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, sales figure, 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 the food-truck owner 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 demand report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised demand report with all four answers, their brief interpretations, and your message to the food-truck owner — written, or recorded as audio or video with a transcript, or as a labelled diagram that describes the original and shifted demand curves 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 3 skills — building quantities from a demand rule, reading the law of demand, telling a change in quantity demanded from a change in demand, naming a shift's direction and determinant, adding buyers into market demand, and explaining the difference 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 the food-truck owner. 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 demand 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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