5 Maths Mistakes That Cost Marks in the CSSE 11+

Why small mistakes matter

The CSSE maths paper rewards accuracy as much as ability. A child who knows every topic can still drop 10–15 marks through rushed working, misread questions or missing units. The good news is that these habits are easy to fix with the right practice.

1. Not reading the whole question

Children often spot a familiar number and start calculating before finishing the sentence. The question might ask for the difference, not the total, or for the answer in centimetres, not metres.

Fix: Underline what the question is actually asking for before writing anything.

2. Forgetting units

A correct number with the wrong unit (or no unit) can lose the mark.

Fix: Finish every answer by checking: Does this need a unit? Is it the right one?

3. Skipping working

In multi-mark questions, working can earn method marks even when the final answer is wrong.

Fix: Write every step, even the ones that feel obvious.

4. Mixing up area and perimeter

Fix: Ask "am I going around or inside?" before choosing a formula.

5. Running out of time

Spending four minutes on one tricky question can cost three easy ones at the end.

Fix: Practise under timed conditions. If a question takes more than two minutes, mark it and move on.

A simple checking routine

Before handing in, spend the last few minutes on:

1. Unanswered questions — make a sensible attempt
2. Units on every answer
3. Any answer that looks unreasonable (a child 4 metres tall?)

Practise with MathsEleven

Our practice papers are written to match the style and difficulty of the real CSSE exam, with full written solutions so your child can learn from every mistake.