Skills

Where Curves Cross

Where two curves cross, they agree on one price and one quantity.

Practice 4: Graphing and Visuals

Part 1 · Hook

Why this matters

At a farmers market, the price of strawberries settles each morning at the point where what shoppers want matches what growers brought. When a heat wave cuts the harvest, that meeting point moves. Most exam graphs ask the same thing: where do the curves cross, and where do they cross after one of them moves?

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Buyers' incomes rise and they want more of a good at every price. On the graph, D

  1. shifts right
  2. shifts left
  3. stays put while you move along it
  4. becomes vertical
Show the answer

A change off the axes shifts the curve; more wanted at every price is a shift right.

  • Correct: shifts right:
  • shifts left:
  • stays put while you move along it:
  • becomes vertical:

2. An upward-sloping curve shows that as price rises, quantity

  1. rises
  2. falls
  3. stays the same
  4. first rises then falls
Show the answer

Upward sloping means both rise together.

  • Correct: rises:
  • falls:
  • stays the same:
  • first rises then falls:

Part 4 · See it

See it first

A downward curve D crosses an upward curve S1 at point E1 (P1, Q1); after the upward curve shifts left to S2, the new crossing E2 has a higher price P2 and a smaller quantity Q2.
The curves cross at E1. After S shifts left to S2, they cross at E2: a higher price (P2) and a smaller quantity (Q2). LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Two curves cross at one pointthat point gives the one price and quantity both curves agree on (E1)
  2. One curve shiftsthe crossing moves along the other curve to a new point (E2)
  3. You draw dashed lines from E1 and E2 to both axesyou can compare the old and new price and quantity
  4. You state each change with its direction"the price rises from P1 to P2" earns the point

Part 6 · Key ideas

Key ideas

  • The intersection point is where two curves cross: on a market graph, the price at which the quantity buyers want equals the quantity sellers offer.
  • When one curve shifts, the crossing slides along the curve that did not move.
  • A before-and-after comparison marks both crossings, E1 and E2, with dashed lines to both axes (P1, P2, Q1, Q2) and says which way each moved.

Part 7 · Misconception

A common mistake

The wrong idea: When a curve shifts right, the new crossing is always higher.

What actually happens: It depends on which curve moved. D shifting right slides up the upward curve S (higher price); S shifting right slides down the downward curve D (lower price). Follow the curve that did not move.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Buyers want more umbrellas at every price after a rainy forecast. Shift the right curve and mark the new point where the curves cross.

QuantityPriceDSQP

Work on the graph (it needs JavaScript), then check.

Show the answer

D shifts right along the upward curve S, so the new crossing E2 is higher and to the right: a higher price and a larger quantity.

  • Key: demand shifts right

2. A new machine lets bakers make bread more cheaply, so at every price they offer more loaves. Shift the right curve and mark the new crossing.

QuantityPriceDSQP

Work on the graph (it needs JavaScript), then check.

Show the answer

S shifts right down along D, so E2 is lower and to the right: a lower price and a larger quantity.

  • Key: supply shifts right

3. On a market graph, what does the point where the two curves cross show?

  1. The highest price buyers will pay for the good
  2. The price at which the quantity buyers want equals the quantity offered
  3. The lowest price sellers will accept for the good
  4. The price at which buyers and sellers trade nothing
Show the answer

At the crossing both curves give the same quantity at the same price, so buyers and sellers agree.

  • The highest price buyers will pay for the good: Buyers would pay more for the first few units; the crossing is where the curves meet.
  • Correct: The price at which the quantity buyers want equals the quantity offered: Right: the two quantities match there.
  • The lowest price sellers will accept for the good: Sellers would accept less for the first few units; the crossing is where the curves meet.
  • The price at which buyers and sellers trade nothing: The crossing is where trade happens, not where it stops.

Data

Buyers and sellers of phone cases

Phone cases buyers want and sellers offer each week at four prices (invented data for practice)
Price ($)Cases buyers wantCases sellers offer
108020
156040
205050
254060

4. At which price in the table do the two curves cross?

  1. $15
  2. $20
  3. $25
  4. $10
Show the answer

At $20 buyers want 50 cases and sellers offer 50, the only row where the two quantities match.

  • $15: At $15 buyers want 60 but sellers offer only 40.
  • Correct: $20: Right: 50 wanted and 50 offered.
  • $25: At $25 sellers offer 60 but buyers want only 40.
  • $10: At $10 buyers want 80 but sellers offer only 20.

5. Using the table, at a price of $10, how many more cases do buyers want than sellers offer each week?

Type a number. A four-function calculator is allowed, as on the exam.

Show the answer

At $10 buyers want 80 and sellers offer 20: 80−20=6080 - 20 = 60 more cases wanted than offered.

  • Answer: 60

6. Sellers find a cheaper supplier, and at each price they now offer 20 more cases than in the table. At what price do the curves cross now?

  1. $15
  2. $20
  3. $25
  4. $10
Show the answer

With 20 more offered, sellers offer 60 at $15, which matches the 60 buyers want there.

  • Correct: $15: Right: 60 wanted and 60 offered at $15.
  • $20: At $20 sellers would now offer 70 but buyers want 50.
  • $25: At $25 sellers would now offer 80 but buyers want 40.
  • $10: At $10 sellers would now offer 40 but buyers want 80.

Part 9 · Summary

Summary

Where two curves cross, they agree on one price and one quantity. When one curve shifts, the crossing slides along the other curve to a new point. Mark both crossings with dashed lines to both axes, label the old and new values, and state the direction of each change: that before-and-after comparison is what readers score on every graph in the course.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Question set for this topic