The 10 most common 11+ maths mistakes — and how to fix each one

Mistake 1: Misreading the question

A child reads quickly, fixates on the numbers, and answers a slightly different question from the one that was asked. Key words like not, least, how many more, altogether, which of these or estimate get skipped.

Why children make this mistake

Time pressure creates a rush to start calculating. The brain sees familiar numbers and jumps ahead. This is one of the most frustrating error types because the working is often fine — the child simply answered the wrong thing.

Example

"Which of the following is NOT a factor of 36?" — A child lists factors of 36 and confidently circles 9. The task was to find a number that is not a factor, so a correct choice might be 5 or 7, not 9.

The fix

Teach your child to read every question twice and underline the key instruction before writing anything. After calculating, ask: "Does my answer actually answer what was asked?" This 10-second habit catches a surprising number of lost marks.

Mistake 2: Skipping units or mixing measurements

A child calculates correctly but forgets units in the answer, or mixes centimetres and millimetres, grams and kilograms, or minutes and hours in one go without converting first.

Why children make this mistake

Children focus on the calculation; the unit feels like an afterthought. In mixed-unit problems, skipping conversion can produce a completely wrong number.

Example

A ribbon is 1.2m long and cut into 15cm pieces. How many pieces? Without converting 1.2m to 120cm, a child might write 1.2 ÷ 15 — instead of 120 ÷ 15 = 8.

The fix

Before starting: convert everything to the same unit and write it clearly. After calculating: check the unit label is in the answer. A good habit is to note the unit in brackets beside each line of working.

Mistake 3: Times tables gaps under timed conditions

A child can recite tables in order but hesitates on isolated facts under pressure — especially 6×7, 7×8, 8×9, and 6×8. That costs time; in multi-step questions a wrong recall compounds through every following step.

Why children make this mistake

Reciting in sequence isn't the same as instant recall. Under exam conditions the "hard facts" in the 6, 7, 8, and 9 times tables are where automaticity often breaks first.

Example

A problem needs 48 × 7 mid-calculation. Hesitation on 8×7 (e.g. 54 instead of 56) pollutes the rest of the answer — even when the child understood the structure of the question.

The fix

Use a 60-second random test to find weak facts, then drill only those — not the whole table — for a couple of minutes most mornings. Flashcards or a simple app is fine. Aim for sub-second recall. Targeted practice for a few weeks usually closes even stubborn gaps. You can also use Studoo's tools such as the times tables speed test.

Mistake 4: Adding fractions without a common denominator

When adding or subtracting, a child adds numerators and denominators: so 1/3 + 1/4 becomes 2/7. The rule for multiplying fractions (top × top, bottom × bottom) is misapplied to addition.

Why children make this mistake

That multiplication pattern feels intuitive. Without a visual model, finding a common denominator can feel arbitrary.

Example

"Calculate 2/5 + 1/3." A child writes 3/8. The right approach is a common denominator (e.g. 15): 6/15 + 5/15 = 11/15. A quick estimate also helps: 2/5 is under half and 1/3 is a bit more than a quarter — the sum can't be 3/8, which is smaller than 2/5 alone.

The fix

Use fraction bars or number lines to show why you can't add fifths and thirds directly. Add a one-line estimate before you calculate, and after: "Is my answer larger than the largest starting fraction?"

Mistake 5: Confusing perimeter and area

A child uses the area idea for perimeter, or the other way around — or uses the right words but the wrong operation (e.g. multiplying all side lengths for perimeter).

Why children make this mistake

Perimeter and area use the same shape and numbers; under time pressure it's easy to blur them.

Example

"Find the perimeter of a rectangle 8cm × 5cm." A child writes 40 — and labels it cm², or adds only 8 + 5, forgetting the opposite sides. Perimeter is distance around the shape; area is the space inside.

The fix

Use two short definitions: perimeter = the path an ant walks around the edge (add the sides). Area = square tiles on the inside (length × width for rectangles). Check the unit: cm vs cm² tells you what you actually calculated.

Mistake 6: Errors with negative numbers in sequences

When a sequence decreases past zero, some children falter: wrong step size, losing the negative sign, or acting as if numbers "stop" at zero.

Why children make this mistake

A full number line that includes negatives is more abstract. Subtracting to go further left of zero needs a clear mental model.

Example

Sequence: 5, 1, -3, -7, __. The step is -4 each time, so the next term is -11. A child might add 4 instead of subtract 4, or give -4 without a minus, because the pattern wasn't tracked on a line.

The fix

For any sequence that crosses zero, draw a number line, mark the given terms, and count equal hops. Ask: "Am I moving to smaller numbers (more to the left)?" That makes the sign and the step size visible.

Mistake 7: Ratio — confusing the part with the whole

A child finds the value of one "part" but gives that as the final answer, or muddles ratio with a fraction of the total.

Why children make this mistake

Ratio language is easy to read quickly wrong: 2:3 means 5 parts in total, and the two people get 2 and 3 of those parts — not "one part each".

Example

"Share £60 in the ratio 2:3." One part = £12. The shares are 2×£12 and 3×£12 — so £24 and £36, not two answers of £12. Check: £24 + £36 = £60.

The fix

Write every step: total parts, value of one part, multiply by each part of the ratio, then a quick add-back check. That verification line catches most ratio slip-ups.

Mistake 8: Miscounting sequence terms

A child spots the pattern but returns the (n-1)th or (n+1)th term because they count jumps in their head and lose track of position.

Why children make this mistake

You need the rule and the index; under pressure, one of them slips.

Example

4, 7, 10, 13, 16… "What is the 8th term?" A child "jumps" from the 4 in the first term and off-by-ones, landing on 22 instead of 25. A two-row table (position / value) keeps the 8th term unambiguous.

The fix

For anything beyond a handful of terms, use a table: position 1,2,3… in one row and the sequence values in the other, and work along to the term you need. Don't count long jumps in your head alone.

Mistake 9: Not checking whether the answer is plausible

A wildly implausible answer (a room 500m long, a car for 4p, 130% when it can't be) gets written down with no second thought.

Why children make this mistake

Children are trained to trust the mechanics. A quick "does that make sense?" is a habit you have to teach; it does not always appear on its own in exam mode.

Example

A 2-hour journey at 60mph — a decimal slip gives 1.2 miles. Two hours on a motorway is clearly not a mile. A one-line ballpark (about 120 miles) would have flagged it instantly.

The fix

Before calculating, jot a rough estimate. After, compare. If the answer is an order of magnitude out, go back. Estimation is the fastest form of error checking you have.

Mistake 10: Poor time allocation — rushing the final section

Too long on early items means rushed or blank later ones. Error rates in the last quarter of a paper are often two to three times those at the start.

Why children make this mistake

Sticking on a hard question feels necessary; the opportunity cost of time is invisible. Without a pacing plan, minutes vanish.

Example

40 questions in 45 minutes ≈ just over a minute each. Four minutes on one early question is roughly the time for three others — and those might have been easy marks.

The fix

Use a two-pass pattern: first pass, answer what you can quickly; don't let one question absorb more than about 60–90 seconds without a good reason. Second pass, use remaining time for the harder ones. Practise that way every time you do timed work, not only in the real exam.

The pattern behind the patterns

These mistakes cluster around three root causes. Addressing the cause is often easier than re-teaching a whole topic from scratch.

• Rushing. Misreading, skipping plausibility checks, and running out of time (mistakes 1, 9, 10) are all about pace. The fix is usually a few seconds of deliberate "slow down" per question, not a huge slowdown overall.
• Shaky fluency. Times tables and fraction rules (3, 4) need short, consistent drilling until they are automatic — not the occasional long revision block.
• No verification step. Units, perimeter vs area, ratio, sequences (2, 5, 7, 8) are the kinds of errors a written check often catches. Build "one line at the end" into the routine in practice until it happens without prompting.

The most effective 11+ maths work doesn't just repeat questions — it builds the habits and self-awareness that stop these errors before they get to the mark scheme. A child who asks "does this make sense?" after every answer, writes working clearly, and can name their times-table weak spots in plain English is in a much stronger place than a child with more "raw ability" but no error awareness.

Using this list with your child

Don't tackle all ten at once. Pick the two or three mistakes you see most on recent papers and focus on those fixes only. In three or four weeks, go through a fresh paper and reassess. A narrow, targeted plan usually moves marks more than unfocused "do more maths" time.

For a longer view of when to start building skills and when to add exam-style work, our year-by-year 11+ preparation guide is a good companion. If you are still new to the exam overall, start with what the 11+ is and the local exam board guide so practice matches the papers you'll actually get.