Every option below carries its key, the reason it earns that key, and the arithmetic in full. This is the part that is normally invisible: prep sites publish a question and name an answer, and vendors publish neither the working nor what a wrong answer means.
Scenario 1 · Percentage points versus relative change
A support team's first-contact resolution rate rose from 64% to 72% over a quarter.
By how much did first-contact resolution improve?
| Option | Key | Why it earns that weight |
|---|
| 8 percentage points, which is a 12.5% relative increase | CorrectKeyed answer | Correct, and it names both quantities. These are different numbers describing the same movement, and a report that gives only one of them is ambiguous. |
| 8% | IncorrectDiagnostic distractor | The most common error in business reporting: a difference in percentage points reported as a percentage change. It is out by more than a third here, and the gap widens as the base falls. |
| 11.1% | IncorrectDiagnostic distractor | Right method, wrong base — 8 ÷ 72 instead of 8 ÷ 64. Diagnoses someone dividing by the new value rather than the starting value. |
| 112.5% | IncorrectDiagnostic distractor | The ratio of new to old read as the increase. Diagnoses a candidate who computed 72 ÷ 64 correctly and then did not subtract the original whole. |
The working. 72 − 64 = 8 percentage points. 8 ÷ 64 = 0.125, so a 12.5% relative increase. Note 8 ÷ 72 = 11.1%, which is the same movement measured against the wrong base.
What the item separates. The item separates candidates who can compute a percentage from candidates who know which base the question is asking about. Only the second group can be trusted with a dashboard.
Scenario 2 · Growth compounds
Revenue grew 10% in the first year and a further 20% in the second.
What was the total growth across the two years?
| Option | Key | Why it earns that weight |
|---|
| 32% | CorrectKeyed answer | Correct. Successive growth multiplies rather than adds. |
| 30% | IncorrectDiagnostic distractor | The additive error — the single most documented miscalculation in numerical reasoning banks. It understates every multi-period growth figure. |
| 15% | IncorrectDiagnostic distractor | The mean of the two rates. Diagnoses a candidate who reached for an average because two numbers were present. |
| 33.1% | IncorrectDiagnostic distractor | Compounding the wrong rate — 1.10 applied three times. Diagnoses someone who has understood compounding and misapplied it, which is a different and more recoverable error than the additive one. |
The working. 1.10 × 1.20 = 1.32, so 32%. The additive answer, 10 + 20 = 30%, omits the 10% earned on the second year's growth. 1.10³ = 1.331 is where 33.1% comes from.
What the item separates. Distractors 2 and 4 are both wrong and they diagnose opposite problems. A report that says only 'incorrect' loses that distinction.
Scenario 3 · Successive discounts
A list price is reduced by 20%, and the reduced price is then cut by a further 15% in a clearance.
What is the total discount against the original list price?
| Option | Key | Why it earns that weight |
|---|
| 32% | CorrectKeyed answer | Correct. The second discount applies to 80% of the list price, not to the list price. |
| 35% | IncorrectDiagnostic distractor | The two discounts added. The same additive error as item 2, in the direction that overstates rather than understates. |
| 68% | IncorrectDiagnostic distractor | The fraction of the price that remains, reported as the discount. Diagnoses a candidate whose arithmetic was right and who then answered a different question. |
| 30% | IncorrectDiagnostic distractor | The most interesting wrong answer here: someone who knows the additive figure overstates and has shaved it by eye. The instinct is right and the correction goes too far — the true answer is 32%, not below 32%. |
The working. 0.80 × 0.85 = 0.68 of the list price remains, so the discount is 1 − 0.68 = 0.32, or 32%. Adding 20 + 15 gives 35% and double-counts the 15% on the fifth of the price already removed.
What the item separates. Every option here is produced by a real method. That is what a good distractor set costs to build, and it is why the wrong answers are worth reading.
Scenario 4 · Averages need weighting
A team of 12 has an average tenure of 3 years. Four people leave; their average tenure was 6 years.
What is the average tenure of the 8 who remain?
| Option | Key | Why it earns that weight |
|---|
| 1.5 years | CorrectKeyed answer | Correct. Work in totals, then divide by the headcount that is actually left. |
| 1 year | IncorrectDiagnostic distractor | The right numerator over the wrong denominator — the remaining tenure divided by the original headcount of 12. Diagnoses a candidate who set the problem up correctly and lost the last step. |
| 4.5 years | IncorrectDiagnostic distractor | The unweighted mean of the two averages. Diagnoses someone averaging averages, which is only valid when the groups are the same size and they are not. |
| 3 years | IncorrectDiagnostic distractor | The assumption that an average is unchanged by who leaves. Diagnoses a candidate treating the mean as a property of the team rather than of the people in it. |
The working. Total tenure = 12 × 3 = 36 years. Leavers = 4 × 6 = 24 years. Remaining = 36 − 24 = 12 years across 8 people = 1.5 years. Dividing that 12 by the original 12 people gives the 1-year distractor.
What the item separates. This is the item that most often separates people who are fluent with percentages but have never had to reconstruct a total from an average — which is most of what workforce reporting asks for.
Scenario 5 · Rates do not average
A courier drives a delivery route at an average 40 km/h and returns along the same route at an average 60 km/h.
What is the average speed for the round trip?
| Option | Key | Why it earns that weight |
|---|
| 48 km/h | CorrectKeyed answer | Correct. More time is spent on the slower leg, so the average sits below the midpoint. |
| 50 km/h | IncorrectDiagnostic distractor | The arithmetic mean of the two speeds — the single most documented wrong method in the rate family. It is the answer most candidates produce in under five seconds. |
| It cannot be determined without knowing the distance | IncorrectDiagnostic distractor | A tempting and wrong caution. The distance cancels, so the answer is the same for a 10 km route and a 1,000 km one. Diagnoses a candidate who has correctly noticed that something is missing and not tested whether it matters. |
| 100 km/h | IncorrectDiagnostic distractor | The two speeds summed. Rare, and when it appears it usually indicates the candidate was out of time rather than out of method — which is exactly the confound the speeded/power distinction above is about. |
The working. For a one-way distance d, time out = d/40 and time back = d/60, so total time = d(1/40 + 1/60) = d/24. Total distance = 2d. Average speed = 2d ÷ (d/24) = 48 km/h. The d cancels, which is why the distance is not needed.
What the item separates. The best-designed items are ones where the fast intuitive answer is on the page. An item whose wrong options are implausible measures reading, not reasoning.