“More units were bought” does not establish which of those happened. The explanation needs the question's assumptions and evidence about the changing factor.
Compare quantities at the same price
This original hypothetical classroom model uses price P in dollars per item and quantity Q in items per week. It is not estimated demand for a real product. Restrict the comparison to prices from 0 through 20 dollars, where both schedules below remain nonnegative.
Original demand D0: Q = 100 - 5P
Changed demand D1: Q = 120 - 5P
| Price per item | Quantity under D0 | Quantity under D1 |
|---|---|---|
| $8 | 60 per week | 80 per week |
| $10 | 50 per week | 70 per week |
| $12 | 40 per week | 60 per week |
At the same $10 price, the schedules give 50 and 70 items per week. D1 represents a rightward demand shift relative to D0: at each listed price, quantity demanded is 20 items greater. The comparison holds price fixed while changing the demand relation.
The equations stipulate this difference. They do not establish why a real market would shift. If a problem attributes it to a preference change, preserve that supplied cause; if it supplies no cause, leave it unspecified.
Follow an own-price change along one schedule
Keep D0 fixed and lower price from $10 to $8. Quantity demanded rises from 50 to 60 per week. That is a movement along D0, because the same equation describes both points.
OpenStax's equilibrium-analysis chapter, checked October 24, 2026, warns against confusing changes in quantity demanded with shifts of demand. A good's own-price change does not itself shift that good's demand curve.
Do not explain the D0 movement by saying that demand increased because price fell. Say that quantity demanded increased along the unchanged demand schedule. This wording preserves which relationship the model holds fixed.
Keep a demand schedule separate from an equilibrium claim
Neither equation supplies a supply schedule. The table therefore cannot identify a market-clearing price or predict the final traded quantity. A demand curve gives desired quantities at possible prices under its assumptions; it does not prove that every listed quantity will be transacted.
If supply also changes, identify that change separately. A single observed price-and-sales pair generally does not reveal which curves shifted. Ask what extra information the problem gives instead of assigning a cause from the direction of two numbers.
Copy a change-first prompt
OpenAI's prompting documentation recommends supplying context and a clear requested result. For economics, include the exact problem text so words such as “price,” “income” and “preference” stay attached to the correct actor and good.
Help me reason through this original demand exercise.
P is dollars per item; Q is items per week; 0 <= P <= 20.
D0: Q = 100 - 5P. D1: Q = 120 - 5P.
No supply schedule or real market observations are supplied.
Compute quantities at P = 8, 10 and 12 under both schedules.
Explain the D0-to-D1 comparison holding price at 10.
Separately keep D0 fixed and change price from 10 to 8.
For each case, identify what changed and whether the answer is
a shift in demand or a change in quantity demanded along a curve.
Do not invent the cause of D1, an equilibrium price or actual sales.
Ask me to describe the two cases in my own words, then check whether
I kept the unchanged relation and the changed variable distinct.
The opportunity-cost activity asks which feasible alternative is forgone when a scarce slot is used. That is a different question from the demand relationship here. Before finishing your diagram, label the curve D0 or D1, put units on the axes, and state exactly which condition changed between the points you are comparing.