IB Economics SLTopic 5 — Assessment and Exam SkillsPaper 2Exam skill~8 min read
Getting Calculation Questions Right
Calculation questions are the most winnable marks on Paper 2, and the most casually thrown away. Almost nobody loses them because the maths is hard. People lose them by writing the answer with no working, dropping a unit, rounding halfway through, or forgetting that a negative sign means something.
📘 What you need to know
Calculations sit in Paper 2 and are the clearest example of AO4: use and application of skills.
They reward method, accuracy and interpretation. Speed is not one of them.
Method marks exist. A correct formula and clear substitution can score even when the final number is wrong.
Always write the formula first, then substitute, then give the answer with its unit.
Round the final answer to two decimal places unless the question says otherwise. Never round in the middle.
Signs matter in elasticity. A negative PED is information, not a typing error.
An answer you calculate early is often evidence you are meant to reuse later in the same question.
How a calculation earns its marks
Examiners are reading for a method they can follow. If the numbers appear with nothing around them, there is nothing to give credit to when something goes wrong. Set every calculation out the same way and you make the marks easy to award.
Line 4 is the one students leave out. Many calculation questions are followed by a short “comment on your answer” part, and that part is where the number becomes economics.
🧩 The rules that apply to every calculation
Show all working. Not for neatness — for method marks.
Formula before numbers. It proves you knew the method even if the arithmetic slips.
Units in the final answer. Dollars, per cent, units of output, or nothing if the value has no unit.
Round once, at the end, to two decimal places unless told otherwise.
Carry the answer forward. If a later part says “using your answer to part (a)”, that is an instruction.
WORKED EXAMPLE
The price of a good rises from $48 to $54. Calculate the percentage change in price. [2]
Step 1: formulapercentage change = (new − old) ÷ old × 100Step 2: substitute= (54 − 48) ÷ 48 × 100= 6 ÷ 48 × 100Step 3: answer with the unitPrice rose by 12.5%Always divide by the old value. Dividing by 54 gives 11.11%, which is the wrong answer to a question nobody asked.
Elasticity: where the signs live
Elasticity questions are two percentage changes and one division, so the maths is easy. The marks go missing in the details: the order of the fraction, the negative sign, and rounding the top and bottom before you divide them.
WORKED EXAMPLE
When the price of a good rises from $8 to $9, quantity demanded falls from 250 to 200 units. Calculate the price elasticity of demand. [4]
Step 1: formulaPED = % change in quantity demanded ÷ % change in priceStep 2: the top of the fraction(200 − 250) ÷ 250 × 100 = −20%Step 3: the bottom of the fraction(9 − 8) ÷ 8 × 100 = 12.5%Step 4: divide, keeping the signPED = −20 ÷ 12.5 = −1.60PED = −1.60Round 12.5 up to 13 first and you get −1.54. That is why rounding early costs marks.
PED has no unit, because it is one percentage divided by another. It does have a sign, and the sign is telling you the law of demand is holding: price up, quantity down. Write it.
Judging the size is a separate skill from calculating it. Questions that ask you to comment on your answer are testing exactly this step.
WORKED EXAMPLE
Using your answer above, comment on the elasticity of demand for this good. [2]
Step 1: quote your own number
PED is −1.60.
Step 2: say what the size means
The size is greater than 1, so demand is price elastic: quantity demanded changes proportionately more than price.
Step 3: take it one step further
Revenue before the price rise was 8 × 250 = $2,000. After it was 9 × 200 = $1,800.
Revenue falls by $200, as elastic demand predictsChecking the revenue is not compulsory, but it shows the number meant something to you.
Treat a calculation as evidence, not as a task you have finished with. The number you worked out in an early subpart is often the strongest thing you can quote in the long evaluation at the end.
The slips that cost the most
The slip
What it costs
The fix
Final answer only, no working
Every method mark, if the answer is wrong
Formula, substitution, answer, every time
No unit on the answer
The accuracy mark
Ask what the number is measured in
Rounding partway through
The final value is slightly off
Keep full figures until the last line
Dropping a negative sign
Elasticity marks, and the interpretation
Write the sign as you calculate it
Dividing by the new value
The whole calculation
Percentage change is always over the old value
Never using the answer again
Application and evaluation marks later
Quote your own figure in the long answer
💡 Exam tip
Write the formula even when you can do it in your head. It is the cheapest mark on the paper.
Label the two percentage changes in an elasticity answer so the examiner sees which is which.
Check the units in the question, not the ones you expected. Millions, thousands and per cent are easy to misread.
If your PED comes out positive for a normal demand curve, you have made a sign error somewhere. Go back.
Underline your final answer so it cannot be missed among the working.
Move on if you are stuck. The method you have written down is already earning marks; the essay at the end is worth more.
⚠️ Common mix-up
Percentage change and percentage points. A rate moving from 8% to 6% falls by 2 percentage points, which is a 25% fall.
Putting the fraction upside down in elasticity. Quantity is on top, price is underneath.
Assuming a negative sign is a mistake. For PED it is normal and expected.
Treating “elastic” as “big demand”. Elastic means responsive to price, not large.
Rounding to two decimal places too early, then again at the end, so the error compounds.
Finishing at the number. If the question asks you to comment, the number is only half the answer.
Up next: Structuring a Paper 1 Essay — how the 10-mark and 15-mark halves of a Paper 1 question are built, paragraph by paragraph.
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