10 Common Mistakes in Selective Maths (and How to Fix Them)
In the Selective and OC Mathematical Reasoning sections, most lost marks don't come from not knowing the maths — they come from a handful of avoidable slips under time pressure. Here are the ten that cost the most, and how to fix each one.
1. Answering the wrong question
The question asks for the number who did NOT pass, or the perimeter not the area, or the second-largest not the largest. Fix: underline exactly what is asked, then re-read your answer against it before moving on.
2. Confusing 'percentage of' with 'percentage change'
A price rises 20% then falls 20% is not back to the start. Fix: for a change, apply it to the new amount each time; for a reverse percentage, divide by the multiplier (÷1.2 for a 20% rise), never just subtract.
3. Forgetting to add the parts in a ratio
Sharing $48 in 3:5 is not $48÷3 and $48÷5. Fix: add the parts first (3+5=8), find one part ($48÷8=$6), then scale.
4. Unit slips
Mixing cm and m, minutes and hours, or writing cm when the answer is cm². Fix: write the unit next to every number, and convert everything to the same unit before calculating.
5. Area vs perimeter
Using length×width when the question wants the distance around. Fix: perimeter = add all sides; area = space inside. Say which one you need out loud.
6. Careless arithmetic under time pressure
13×7 becomes 81, or a minus sign is dropped. Fix: estimate first (13×7 ≈ 90), so a wrong answer looks wrong immediately.
7. Not eliminating options
Staring at all five choices instead of ruling out the impossible ones. Fix: cross out clearly-wrong options first — often two vanish instantly, and a good guess beats a blank.
8. Spending too long on one hard question
Burning four minutes on question 12 and missing three easy ones at the end. Fix: if you're stuck after ~90 seconds, mark it, move on, and come back.
9. Misreading tables, graphs and diagrams
Reading the wrong bar, missing the scale, or assuming a diagram is to scale. Fix: check the axis labels and scale first, and trust the numbers over the picture.
10. Not checking the answer is sensible
An 'average age' of 200, or a probability greater than 1. Fix: in the last five seconds per question, ask 'could this actually be true?'
How to practise fixing these
The fastest way to stop making these mistakes is deliberate practice: after every wrong answer, name which of the ten it was. Over a week or two the pattern becomes obvious — and that's the one to drill. PrepPath explains every question in full and has an AI Tutor that guides you through the method when you're stuck.
Practise now with free Selective and OC Mathematical Reasoning — or see worked examples in our Selective sample questions and OC Maths questions.
PrepPath is an independent educational platform and is not affiliated with ACARA or the NSW Department of Education. This guide is general information only — always check the official ACARA (nap.edu.au) or NSW Department of Education websites for current dates and rules.