Week 4 · Model Workshop

Week 4 Model Workshop: The Night-Market Bubble Tea Stalls

Principles of Microeconomics Free sample edition

Part 1 — The Big Picture

This week you learned that sellers offer more of a good when its own price rises, that the market price settles where the amount buyers want equals the amount sellers offer, and that every change in a market moves one curve or both. In this workshop you build both sides of a small market, bubble tea at a weekend night market, in Desmos or a spreadsheet, find where the two curves cross, read what happens when the price sits too low or too high, and then follow four changes to their new price and quantity. Then you catch a chatbot's slips on the same market. The habit you build here, name the curve, then the direction, then the new price and quantity, is the one every later week leans on.

Time budget: Plan for about 90 minutes.

Part 2 — The Task

Guiding question: When something changes at the night market, how do you tell which curve moved and which way, and where do the price and the number of cups sold end up?

Workshop data: the illustrative night-market bubble tea schedule

Source: A small illustrative schedule constructed for this workshop, not real data: buyers' demand and the stalls' supply of bubble tea at a weekend night market, in cups per hour, each given by a rule. Every value you need is printed in Part 4.

Rows: 5 prices from 3 to 7 dollars, each with a quantity demanded and a quantity supplied (the scaffold tables have 5 and 4 rows)
Columns: price in dollars, quantity demanded, quantity supplied, and the gap between them, in cups per hour
Rules: buyers want 160 − 20P cups an hour and the stalls offer 20P − 40 cups an hour

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 3, 4, 5, 6, and 7 in cells A2 to A6, and label row 1 as Price, Demand, Supply, and Gap. Tool: Desmos (free) or Google Sheets (free). Check: Five prices sit in A2 to A6 under the label Price.
In B2 type =160-20*A2 and in C2 type =20*A2-40, then copy both formulas down to row 6. In D2 type =B2-C2 and copy it down; a positive gap means buyers want more than sellers offer. Tool: Desmos (free) or Google Sheets (free). Check: Column B falls by 20 for each 1-dollar rise and column C rises by 20; column D changes sign once as the price climbs.
Open Desmos. Quantity goes on the x-axis and price on the y-axis, so type y = 8 - x/20 for demand and y = 2 + x/20 for supply, then click the point where the two lines cross. Tool: Desmos (free) or Google Sheets (free). Check: One line falls from the upper left and the other rises from the lower left, and Desmos labels a single crossing point.
Change supply to y = b + x/20 and accept the slider for b. Start at b = 2, then move it to 4 to show the tapioca shortage and to 0 to show a new stall opening, and watch where the crossing moves. Tool: Desmos (free) or Google Sheets (free). Check: The supply line keeps its tilt and only slides left as b rises or right as b falls, and the crossing slides along the demand line.
Change demand to y = a - x/20 with a slider for a. Set a to 10 to show the heat wave, with b back at 2, then fill in the Part 4 tables from your sheet and your graph. Tool: Desmos (free) or Google Sheets (free). Check: The demand line slides right without changing its tilt, and the crossing moves up and to the right along the supply line.
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, 160 minus 20 times the price). Describe each curve in words instead of drawing it in Desmos: what is on each axis, where each line starts and ends, where they cross, and which way a line moves after each change. 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 both sides of the market at each price and find the gap; this is the schedule every later question reads from.

Student scaffold table 1
Price (dollars)	Quantity demanded: 160 − 20P	Quantity supplied: 20P − 40
3	100	20
4	80
5
6		80
7

Follow each change to its new equilibrium, naming the curve that moved and its direction before you solve.

Student scaffold table 2
Change	Curve that moved and direction	New price (dollars)	New quantity (cups per hour)
A tapioca shortage: supply becomes Qs = 20P − 80
A heat wave: demand becomes Qd = 200 − 20P
A heat wave and a new stall together: Qd = 200 − 20P and Qs = 20P
A heat wave and a tapioca shortage together: Qd = 200 − 20P and Qs = 20P − 80

Part 5 — Analysis Questions

Set quantity demanded equal to quantity supplied and solve for the equilibrium price and quantity, showing each step. Check your result against the crossing point in Desmos and say in one plain sentence what it means for buyers and stalls.
Use your schedule to name what happens at 3 dollars and at 7 dollars, how large each gap is, and which way each gap pushes the price.
A tapioca shortage raises the stalls' cost and supply becomes Qs = 20P − 80. Name the curve that moved and its direction, solve for the new price and quantity, and explain why buyers end up with fewer cups even though their demand did not change.
Compare the last two rows of your second table. For each pair of changes, say which result, price or quantity, both shifts push the same way, and which result would be open if the two shifts were not the same size.
Name one limitation of this model, such as the straight-line rules, the hour-by-hour market settling instantly, or every stall charging one price, and explain how it could change a decision a stall 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 supply and market equilibrium in an introductory microeconomics course. At a weekend night market, in an illustrative example, not real data, buyers' demand for bubble tea is Qd = 160 − 20P and the stalls' supply is Qs = 20P − 40, in cups per hour, where P is the price in dollars. Build the schedule at prices of 3 to 7 dollars and find the equilibrium price and quantity. Then answer: what happens at 3 dollars and at 7 dollars? If the stalls raise their price from 5 to 6 dollars, what happens to supply? If a tapioca shortage changes supply to Qs = 20P − 80, what are the new price and quantity, and what happened to demand? If a heat wave changes demand to Qd = 200 − 20P while a new stall changes supply to Qs = 20P, what happens to price and quantity? Finally, tell me how price-sensitive the buyers are, whether the city should cap the price or tax each cup and who would pay that tax, what price a stall should charge to maximize profit and how much profit the stalls make, whether one stall should make only bubble tea because it is the fastest, and whether the stalls' costs include the litter from their cups.

Test the response for these likely failures:

Chatbots often treat the stall's price rise from 5 to 6 dollars as the whole supply curve shifting right.
After the tapioca shortage raises the price, chatbots often say that demand for bubble tea fell.
Chatbots often fold a value judgment, such as that the market ought to keep the price at 5 dollars so everyone can afford a drink, into the model's result.
Chatbots sometimes say a stall should make only bubble tea because it is the fastest maker at the market.
Chatbots often say the steady change of 20 cups per dollar is the buyers' price elasticity.
Chatbots often label the buyers' demand as elastic or inelastic with no calculation.
Chatbots sometimes call a city cap on drink prices a price floor, or say a cap below equilibrium creates a surplus.
Chatbots often say a per-cup tax paid by the stalls is borne entirely by the stalls.
Chatbots sometimes invent a cost per cup and treat that average as the cost of one more cup.
Chatbots often say a stall should set its price equal to marginal cost to maximize profit.
Chatbots sometimes multiply 5 dollars by 60 cups and call the result the stalls' profit.
Chatbots sometimes claim the stalls' costs already include the litter that plastic cups leave around the market.

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 4. 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 night-market bubble tea stalls, printed here in full: "The market settles at 4 dollars, where the stalls offer 80 cups an hour. At 7 dollars there is a shortage, because the price is high. When the stalls raise their price from 5 to 6 dollars, supply increases and the whole supply curve shifts right. The tapioca shortage made demand for bubble tea fall, so the price dropped to 4 dollars. When the heat wave and the new stall arrive together, both the price and the quantity must rise. The steady change of 20 cups per dollar is the buyers' elasticity, so their demand is elastic. The price ought to stay at 5 dollars, and that is what the model proves. The city should cap the price at 4 dollars, which is a price floor, and a tax paid by the stalls would fall entirely on the stalls. Each stall should price at marginal cost, which is its average cost of 2 dollars a cup, and at 5 dollars the stalls make a profit of 5 times 60, or 300 dollars an hour, which is their economic profit. The fastest stall should make only bubble tea because it is the best at it, and the stalls' costs already include the litter from their cups." Mark every slip, name the failure each shows, correct each one by hand against your own tables and Chapter 4, 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 night-market bubble tea stalls: two completed tables (the schedule with its gaps, and four changes followed to their new equilibrium), written answers to six analysis questions, and a critique of a chatbot's answers about the same market, 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
Schedule built and equilibrium solved	Both quantity columns and the gap column are complete from the rules, the equilibrium is found by setting the rules equal with every step shown and checked on both sides, and a plain sentence says what it means.	The schedule or the algebra is mostly complete, but one or two values are wrong or missing, or the check on both sides is missing.	No schedule or no equilibrium, or the equilibrium is read from one rule alone.
Shifts followed to the new equilibrium	Each of the four changes names the curve that moved and its direction and lands on the correct new price and quantity, and each double shift says which result would be open with unequal shifts.	Two or three changes are followed correctly, or all four are labeled but a price, a quantity, or a direction is wrong.	The changes are not followed, or an input-cost change is read as a demand shift throughout.
Plain-sentence analysis and limitation	All six analysis questions are answered from the market's own numbers, each gap is labeled shortage or surplus with the direction of pressure, the limitation is tied to a decision, and the chosen reflection option is answered with a specific example.	Most questions are answered, but a gap is mislabeled, the limitation is 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 every change, 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 quantity comes from a rule, shown by formula or by hand, never from memory or a chatbot.
Every equilibrium is found by setting quantity demanded equal to quantity supplied, with the algebra shown and checked on both sides.
Every gap away from equilibrium is labeled a shortage below the equilibrium price or a surplus above it.
Every change names the curve that moved, its direction, and the new price and quantity; a double shift says which result would be open with unequal shifts.
Every computed result ends with one plain sentence about what it means for buyers or stalls.
No real-world price, sales 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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