Week 3 · Model Workshop

Week 3 Model Workshop: The Riverside Bike-Rental Stand

Principles of Microeconomics Free sample edition

Part 1 — The Big Picture

This week you learned that buyers want less of a good when its own price rises, that a market's demand adds every buyer's quantity at each price, and that a change in the good's own price slides buyers along the demand curve while a change in anything else moves the whole curve. In this workshop you build a real schedule for a riverside bike-rental stand from two groups of renters, draw it in Desmos or a spreadsheet, and then put the two kinds of change side by side: a price cut and a festival weekend. Then you catch a chatbot's slips on the same stand. The habit you build here, name the cause before you move anything, carries into every model this course builds next.

Time budget: Plan for about 90 minutes.

Part 2 — The Task

Guiding question: When more bikes are rented at the riverside stand, how can you tell whether buyers slid along the same demand curve or the whole curve moved, and what does each change mean for the stand?

Workshop data: the illustrative bike-rental stand's weekend demand

Source: A small illustrative schedule constructed for this workshop, not real data: two groups of renters, locals and visitors, at a riverside bike-rental stand on a weekend day, with each group's demand for one-hour rentals given by a rule. Every value is printed in Part 4.

Rows: 6 prices from 2 to 7 dollars, each with a locals' quantity, a visitors' quantity, and a market quantity (the scaffold tables have 6 and 3 rows)
Columns: price in dollars, locals' quantity, visitors' quantity, and market quantity, in one-hour rentals per day
Rules: locals want 70 − 10P and visitors want 90 − 10P one-hour rentals a day

Part 3 — Materials & Procedure

Tools and materials

Desmos (free) or Google Sheets (free)

Ways of working

build-the-supply-and-demand-schedule
shift-vs-move-along-the-curve
compute-elasticity-by-the-midpoint-formula

Procedure

Open a blank Google Sheets file. In column A, type the prices 2, 3, 4, 5, 6, and 7 in cells A2 to A7, and label row 1 as Price, Locals, Visitors, and Market. Tool: Desmos (free) or Google Sheets (free). Check: Six prices sit in A2 to A7 under the label Price.
In B2 type =70-10*A2 and in C2 type =90-10*A2, then copy both formulas down to row 7. In D2 type =B2+C2 and copy it down. Tool: Desmos (free) or Google Sheets (free). Check: Column D starts at 120 at 2 dollars and falls by 20 for each 1-dollar rise, ending at 20 at 7 dollars.
Open Desmos. Quantity goes on the x-axis and price on the y-axis, so type y = 8 - x/20 to draw the market demand curve, and click the line at x = 60 to read its price. Tool: Desmos (free) or Google Sheets (free). Check: The line falls from the upper left to the lower right and passes through (60, 5).
In Desmos, type y = a - x/20 and accept the slider for a. Set a to 8, then move it to 10 to show the festival weekend, and to 6 to show a rival scooter stand cutting its prices. Tool: Desmos (free) or Google Sheets (free). Check: At a = 10 the line passes through (100, 5); at a = 6 it passes through (20, 5). The line keeps its tilt and only slides right or left.
Back in the spreadsheet, add column E labeled Festival with =200-20*A2 and column F labeled Rival cut with =MAX(0,120-20*A2), copied down; the MAX keeps a quantity from falling below zero. Then fill in the Part 4 tables from your sheet. Tool: Desmos (free) or Google Sheets (free). Check: Column E is 40 above column D in every row, and column F is 40 below it until it reaches 0 at 6 dollars and above.
Answer the analysis questions in Part 5, then do the AI-critique step in Part 6 or its equivalent route.

Fallback route

When to use it: Use this route if you cannot open Google Sheets or Desmos, or your device cannot run them.

Complete both Part 4 tables by hand with any calculator, writing each rule in words beside its result (for example, locals plus visitors at 5 dollars). Describe each demand curve in words instead of drawing it in Desmos: what is on each axis, where the line starts and ends, and which way it moves. Answer all six analysis questions, do the critique step, and submit the same items for equal credit.

This route earns the same credit as the primary route.

Part 4 — The Scaffold

Build the market schedule by adding the two groups' quantities at each price; this is the demand side of the stand's market.

Student scaffold table 1
Price (dollars)	Locals: 70 − 10P	Visitors: 90 − 10P
2	50	70
3	40	60
4	30	50
5	20	40
6	10	30
7	0	20

Compare one change in the stand's own price with two changes in something else, and name each one as a movement along the curve or a shift.

Student scaffold table 2
Change	Market quantity before	Market quantity after	Movement along or shift, and which way
Price falls from 6 to 4 dollars
Festival weekend, price stays at 5 dollars: Qd = 200 − 20P
Rival scooter stand cuts its prices, price stays at 5 dollars: Qd = 120 − 20P

Part 5 — Analysis Questions

Add the locals' and visitors' quantities at each price to complete the market schedule, and check that each 1-dollar rise lowers market quantity demanded by the same amount. Say in one plain sentence what the schedule shows about renters.
The stand cuts its price from 6 to 4 dollars and nothing else changes. Use the schedule to find the market quantity before and after, and describe what happens on the Desmos graph.
On a festival weekend, market demand becomes Qd = 200 − 20P. At a price of 5 dollars, how many rentals are wanted compared with the ordinary weekend? Name the determinant of demand that changed, the curve that moved, and its direction.
A rival scooter stand nearby cuts its prices, and the bike stand's market demand becomes Qd = 120 − 20P. Find quantity demanded at 5 dollars, explain whether bikes and scooters are substitutes or complements for these renters, and say which way the curve moved and why.
Name one limitation of this model, such as the straight-line rules, the two neat groups of renters, or the fact that demand alone cannot say what price the stand will end up charging, and explain how it could change a decision the stand's owner makes.
Choose one reflection option, (a) or (b), and answer it in 3–4 sentences.

Part 6 — AI-Critique Moment

Choose one equal-credit critique route. Route A: ask an approved AI tool to critique your completed work, then check its response against your own evidence. Route B is the no-AI equivalent printed below.

Approved tools: Claude, Gemini, ChatGPT, Grok, Microsoft Copilot.

Prompt to paste:

I am studying demand in an introductory microeconomics course. A riverside bike-rental stand has two groups of renters on a weekend day, in an illustrative example, not real data. Locals want 70 − 10P one-hour rentals and visitors want 90 − 10P, where P is the price in dollars. Build the market demand schedule at prices of 2 to 7 dollars. Then answer: if the stand cuts its price from 6 to 4 dollars, what happens to demand? If a festival brings more visitors so that market demand becomes 200 − 20P, how many rentals are wanted at 5 dollars, and which way did the curve move? Finally, tell me how price-sensitive renters are, whether the city should cap the rental price or tax each rental and who would pay that tax, what price the stand should charge to maximize profit and how much profit it makes, and whether the stand should specialize in rentals because it is the best at them.

Test the response for these likely failures:

Chatbots often draw the price cut from 6 to 4 dollars as the whole demand curve shifting right.
Chatbots often write that the price cut made demand increase by 40 rentals.
Chatbots often mix a value judgment, such as that the city should cap the price so everyone can ride, into the model's result.
Chatbots sometimes say the stand should specialize because it is the best at renting bikes.
Chatbots often say the steady loss of 20 rentals per dollar is the renters' price elasticity.
Chatbots often label the renters' demand as inelastic or elastic with no calculation.
Chatbots sometimes call a cap on the rental price a price floor, or say it raises the price.
Chatbots often say a per-rental tax paid by the stand is borne entirely by the stand.
Chatbots sometimes invent the stand's cost per rental and treat that average cost as the cost of one more rental.
Chatbots often say the stand should set its price equal to marginal cost to maximize profit.
Chatbots sometimes multiply price by quantity and call the result the stand's profit.
Chatbots sometimes claim the stand's costs already include any crowding of the river path that rentals cause.

Write 3–5 sentences naming what the chatbot got right and at least two places where it slipped into one of the failures above, quoting its words and correcting each by hand against your own completed tables and Chapter 3. For any claim about later material, say that it is out of scope this week and why.

Equivalent route (no AI required)

No AI account is required for this workshop. This route assesses the same objectives, Objective 2 (explain how demand and supply determine a market's equilibrium price and quantity, and predict how a shift in demand or supply changes them) and Objective 1 (use the tools of economic modeling — scarcity, opportunity cost, the production possibilities frontier, comparative advantage, and the gains from trade — and distinguish positive statements (what is) from normative statements (what ought to be)), takes comparable effort of about the same 90 minutes, uses the same rubric, and earns the same credit. Instead of the chatbot, critique this flawed analysis of the bike-rental stand, printed here in full: "When the stand cuts its price from 6 to 4 dollars, demand increases by 40 rentals and the whole demand curve shifts right. On the festival weekend, buyers want 60 rentals at 5 dollars, the same as before, because the price did not change. The steady drop of 20 rentals per dollar is the renters' elasticity, so their demand is inelastic. The city should cap the rental price at 3 dollars, which is a price floor, and a tax paid by the stand would fall entirely on the stand. The stand should price at marginal cost, which is its average cost of 2 dollars a rental, and at 4 dollars it makes a profit of 4 times 80, or 320 dollars, which is its economic profit. It should specialize in rentals because it is the best stand on the river, and its costs already include any crowding on the river path." Mark every slip, name the failure each shows, correct each one by hand against your own tables and Chapter 3, and flag each claim about later material as out of scope this week. Submit the marked-up analysis with your corrections in place of the chatbot report and share link.

Part 7 — What to Submit

The two completed scaffold tables, from a spreadsheet, Desmos, or by hand
Written answers to all six analysis questions
The chatbot critique report with its share link, or the equivalent route's marked-up correction of the printed flawed analysis

Deliverable:

A short Model Workshop report on the riverside bike-rental stand: two completed tables (the market schedule built from two groups of renters, and three changes labeled as movements or shifts), written answers to six analysis questions, and a critique of a chatbot's answers about the same stand, or, on the equivalent route, a marked-up correction of the printed flawed analysis. 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. Choose how you show it: written answers, a recorded explanation with a transcript, or a labelled diagram that describes the curves in words with a short note; every form is graded by the same criteria.

Rubric

The rubric below shows how each submitted part is evaluated; bands are full, partial, or none.

Workshop evaluation criteria
Criterion	Full	Partial	None
Market schedule built and summed	All six market quantities are the sum of the two groups' quantities at the same price, shown by formula or by hand, and the steady 20-rental drop per dollar is described in a plain sentence.	The schedule is mostly complete, but one or two sums are wrong or missing, or the plain sentence is missing.	No market schedule, or quantities were added across different prices.
Movements and shifts read correctly	The price cut is read as a movement along the same curve with the before and after quantities, and the festival and the rival's price cut are each read as a shift with its determinant, direction, and quantity at 5 dollars.	Two of the three changes are read correctly, or all three are labeled but a quantity or a direction is wrong.	The changes are not labeled, or a price change is read as a shift throughout.
Plain-sentence analysis and limitation	All six analysis questions are answered from the stand's own numbers, the substitutes question gives a reason, the limitation is named with its effect on a decision, and the chosen reflection option is answered with a specific example.	Most questions are answered, but the limitation is missing or vague, or the reflection is missing.	No analysis answers.
AI critique or equivalent correction	The chatbot's reply, or the printed flawed analysis on the equivalent route, is quoted, at least two slips are named with the failure each shows, each is corrected by hand, and out-of-scope claims are flagged; either route earns this band equally.	One slip is named and corrected on either route, or slips are named without a correction.	No critique on either route, or the chatbot's reply is pasted without any check.
Model built in the tools, or by hand	The spreadsheet formulas or the Desmos lines are shown or described, including the slider values for both shifts, or the fallback's hand work and written curve descriptions are complete.	The model is partly shown, such as the schedule without the shifted curves.	No evidence of how the numbers or curves were produced.

Quality Gate

Every market quantity comes from adding the two groups' quantities at the same price, shown by formula or by hand, never from memory or a chatbot.
Every change is labeled as a movement along the curve or a shift, using the words quantity demanded and demand correctly.
Every shift names its determinant and its direction, right for an increase and left for a decrease.
Every computed result ends with one plain sentence about what it means for the stand or its renters.
No real-world price, rental count, statistic, or study is invented anywhere in the submission.
The critique quotes the chatbot, or the printed flawed analysis, and corrects each slip by hand.

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