Indifference curves and budget lines
Indifference curves and budget lines: build the core model, apply it to evidence and qualify conclusions where outcomes depend on context.
9708 section 7.2; A Level. Checked against Cambridge's 2026–2028 Version 2 (latest version published December 2025) and 2029 Version 1 (published September 2026). Cambridge states that neither version has significant changes affecting teaching. Always follow the syllabus for the candidate's examination year.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a standard indifference curve represent?
What you will learn
- Interpret indifference curves and budget lines
- Explain income and price changes to the budget line
- Separate substitution and income effects
- Analyse normal, inferior and Giffen goods and model limits
Core model and terms
An indifference curve joins combinations of two goods that give the consumer equal satisfaction; curves farther from the origin are preferred under standard assumptions. A budget line shows affordable combinations: its intercepts depend on income and prices, and its slope is the relative price. Higher income shifts it outward in parallel; a price change pivots it around the unchanged-price intercept.
Apply and evaluate
A price change creates a substitution effect toward the relatively cheaper good and an income effect because purchasing power changes. For a normal good, the income effect reinforces the substitution effect; for an inferior good it opposes it. A Giffen good is a rare inferior good whose negative income effect is large enough to outweigh substitution, so a price rise can increase quantity demanded. The model assumes consistent preferences and a two-good budget representation.
Pause and explain
Which statement best applies indifference curves and budget lines?
Worked example
A learner has $60 to spend on books at $12 each and cinema tickets at $6 each. Find the two intercepts and the budget-line slope with books on the horizontal axis.
Show the worked solution
- Books intercept = 60 ÷ 12 = 5
- Ticket intercept = 60 ÷ 6 = 10
- Slope = −12 ÷ 6 = −2 tickets per book
Answer Books intercept = 60 ÷ 12 = 5 → Ticket intercept = 60 ÷ 6 = 10 → Slope = −12 ÷ 6 = −2 tickets per book
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A learner has $60 to spend on books at $12 each and cinema tickets at $6 each. Find the two intercepts and the budget-line slope with books on the horizontal axis.
Give me a hint
Spend the full budget on one good for each intercept; use −Px/Py for the slope.
Compare my reasoning
- Books intercept = 60 ÷ 12 = 5
- Ticket intercept = 60 ÷ 6 = 10
- Slope = −12 ÷ 6 = −2 tickets per book
Books intercept = 60 ÷ 12 = 5 Ticket intercept = 60 ÷ 6 = 10 Slope = −12 ÷ 6 = −2 tickets per book
Look for these in your work
- Books intercept = 60 ÷ 12 = 5
- Ticket intercept = 60 ÷ 6 = 10
- Slope = −12 ÷ 6 = −2 tickets per book
Income rises while both prices stay fixed. Describe the budget-line change.
Give me a hint
Check which intercepts change and whether relative prices changed.
Compare my reasoning
- Purchasing power rises
- Both intercepts increase proportionally
- The budget line shifts outward in parallel
Purchasing power rises Both intercepts increase proportionally The budget line shifts outward in parallel
Look for these in your work
- Purchasing power rises
- Both intercepts increase proportionally
- The budget line shifts outward in parallel
The price of Good X falls while income and the price of Y stay fixed. Explain the substitution effect on X.
Give me a hint
Compare X with Y at the new relative price.
Compare my reasoning
- X becomes cheaper relative to Y
- The consumer has an incentive to substitute toward X
- This effect raises chosen X, other things equal
X becomes cheaper relative to Y The consumer has an incentive to substitute toward X This effect raises chosen X, other things equal
Look for these in your work
- X becomes cheaper relative to Y
- The consumer has an incentive to substitute toward X
- This effect raises chosen X, other things equal
A staple is inferior and its price rises; its quantity demanded also rises in the stated case. What must be true of the income effect?
Give me a hint
Combine the opposing substitution and income effects.
Compare my reasoning
- The substitution effect tends to reduce X
- The income effect must work in the opposite direction and be larger
- This is the Giffen-good case
The substitution effect tends to reduce X The income effect must work in the opposite direction and be larger This is the Giffen-good case
Look for these in your work
- The substitution effect tends to reduce X
- The income effect must work in the opposite direction and be larger
- This is the Giffen-good case
Common mistakes
- Calling every inferior good a Giffen good or treating a budget-line shift as a change in preferences.
What can you explain now?
What does a standard indifference curve represent?
Compare with the explanation
Combinations of two goods that give equal utility
The consumer is indifferent between combinations on the same curve under the model.
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Make it stick
Before moving on, explain the central idea in indifference curves and budget lines and state one condition that could change its effect.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.