Week 3 — Reading the Demand Curve: Slides, Shifts, and Buyers — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

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 week you learned the law of demand, how to build a demand schedule from a rule, how to add buyers into market demand, and how to tell a movement along the demand curve from a shift of the whole curve. In Week 1 you learned that economists hold everything else fixed, ceteris paribus, to see one cause at a time; the law of demand uses exactly that habit. In this assignment you work four short problems on this week's illustrative market, interpret each one, and explain a claim to someone who has never studied economics. This week's lecture tutorial, quiz, discussion, and Model Workshop practice the same ideas from other angles.

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.

Show your arithmetic. A reader should be able to reproduce every number you report from the values above without guessing which values or steps you used.

Deliverables

Submit a single demand report containing:

• the four answers, each with the arithmetic shown and a brief interpretation as the task describes;
• your message to the food-truck owner, recommending a more accurate wording, explaining a slide along the demand curve versus a shift of the whole curve, saying what would signal a real shift, and naming one limit; and
• a one-line restatement of the values you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes 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. 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.

How you'll be graded

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

Academic integrity and AI use

This version is designed to be completed without AI: do the computations, the interpretations, and the writing yourself, without any AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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