Mark a few dozen GCSE maths mock papers and a pattern shows up fast: a meaningful chunk of lost marks have nothing to do with a student not knowing the maths. Here's what actually costs marks.
Not reading what the question is asking
"Give your answer to 2 significant figures." "Show your working." "Give your answer in its simplest form." These instructions are easy to skim past when you're focused on the method — and skimming past them costs marks even when the maths is completely correct.
Not showing method for method marks
GCSE maths gives marks for method, not just final answers. A student who does the calculation in their head and writes down only the answer can lose marks on a question they actually got right, simply because there's nothing on the page to award method marks for.
Rounding too early
Rounding an intermediate answer before using it in the next step of a calculation introduces small errors that compound — and examiners notice. The fix is simple once it's pointed out: keep full precision until the final answer, then round once, at the end.
Not checking whether an answer is plausible
A student calculates that a triangle's side is 400cm and moves on without blinking, even though the triangle in the diagram is clearly a few centimetres across. A five-second sanity check — does this answer make sense given the question? — catches a surprising number of errors that would otherwise go unnoticed until the exam is marked.
The fix isn't more content — it's exam technique
None of this is about knowing more maths. It's about the habits that turn "knows the maths" into "gets the marks." That's a coachable skill, and it's exactly the kind of thing that's easy to miss when practising alone and much easier to catch with someone watching your working.