11+ Maths Common Mistakes & How to Avoid Them

Year 6 students taking the 11+ Maths exam make remarkably consistent mistakes across reasoning, arithmetic, and problem-solving. Many of these errors aren't about lacking mathematical ability — they're about misreading questions, forgetting to check, or misunderstanding what the question is asking for. This guide identifies the top 12 mistakes, shows real examples, and gives you specific strategies to avoid them.

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The 12 Most Common 11+ Maths Mistakes

Careless Errors

Misreading numbers, forgetting zeros, dropping digits during calculation.

Conceptual Errors

Misunderstanding what the question asks; wrong operation applied.

Speed Errors

Rushing through calculations; skipping steps; not checking work.

Reading Errors

Missing key information; ignoring units; not spotting worded clues.

Top 12 Maths Mistakes — How Often They Occur

These 12 mistakes account for ~85% of all 11+ Maths errors. Focus on the ones you make most often.

Mistake 1: Forgetting to Read the Question Fully

❌ The Mistake

Students solve the wrong problem because they don't read all the way to the end of the question.

Example:

"A shop sells apples at 3 for £1. How many apples can you buy with £4? How much change will you get?"

Student answers: "12 apples" and stops.

They calculated the first part but forgot that the question then asks for change.

Why Students Make This Mistake:

In a timed exam, there's pressure to answer quickly. Students spot the first question mark and stop reading. Worded problems often have multiple parts hidden at the end.

Correct Approach:

Step 1: Read the entire question once without writing anything.
Step 2: Underline or highlight what you're being asked to find. Worded problems often ask for more than one thing.
Step 3: Then solve.

In the example: "12 apples costs £4. Change from £4 = £0."

How to Avoid This:

Always read the question twice before answering
Circle or underline what you're being asked to find
Check: Does the question have multiple parts? ("How much... then how much...")
Look for phrases like "altogether", "change", "left over" — these signal a second calculation

Mistake 2: Dropping Zeros or Misplacing the Decimal Point

❌ The Mistake

Writing 120 as 12, or 0.3 as .3, or 3.05 as 3.5. A tiny slip becomes a completely wrong answer.

Example 1 (Zeros):

Question: "5 × 206 = ?"

Student writes: 5 × 206 = 5 × 200 + 5 × 6 = 1000 + 30 = 1030

But they forget the zero and write 103.

Example 2 (Decimals):

Question: "0.3 × 4 = ?"

Student calculates correctly: 0.3 × 4 = 1.2.

But writes down: 12 (forgot the decimal point).

Why Students Make This Mistake:

Zeros and decimal points are "invisible" — they're easy to skip over when writing quickly. The mental calculation might be correct, but the notation is sloppy.

Correct Approach:

Always count your digits before writing the final answer.

For 5 × 206 = 1030:
• Write it out: 1-0-3-0 (4 digits)
• Check: Does this make sense? 5 × 200 = 1000, so 5 × 206 should be >1000. ✓

For 0.3 × 4 = 1.2:
• Write: 1.2 (not 12)
• Check: 0.3 is less than 1, so answer should be less than 4. ✓

How to Avoid This:

Write the final answer in a separate, clear box
Count all the digits you've written (including zeros)
For decimals: say the number out loud before writing ("one point two", not "twelve")
Sanity check: Is the answer reasonable given the question?

Mistake 3: Confusing Fraction, Decimal, and Percentage Conversions

❌ The Mistake

Converting 1/4 to 0.25% instead of 25%, or writing 0.5 as 50% instead of 50.

Example 1:

Question: "Write 1/4 as a decimal and as a percentage."

Student answers: "Decimal: 0.25, Percentage: 0.25%"

(The percentage should be 25%, not 0.25%)

Example 2:

Question: "What is 20% of 50?"

Student calculates: 20% = 0.2, so 0.2 × 50 = 10. (Correct!)

But then writes: "20% of 50 = 0.2"

(They forgot the calculation and just wrote the decimal.)

Why Students Make This Mistake:

Fractions, decimals, and percentages are three different notation systems for the same idea. Students confuse which is which, or forget the relationship between them. They often learn them as isolated tricks ("move the decimal point") rather than as equivalent forms.

Correct Approach — The Conversion Triangle:

Fraction → Decimal: Divide the numerator by the denominator.
1/4 = 1 ÷ 4 = 0.25

Decimal → Percentage: Multiply by 100 (move the decimal point 2 places right).
0.25 × 100 = 25%

Percentage → Decimal: Divide by 100 (move the decimal point 2 places left).
20% ÷ 100 = 0.2

Percentage → Fraction: Put the percentage number over 100, then simplify.
25% = 25/100 = 1/4

Practice Tip: Make a conversion table. Write down: 1/2, 1/4, 1/5, 1/10, 3/4 and their decimal and percentage equivalents. Memorize these five. They appear constantly in 11+ exams.

How to Avoid This:

Know your "magic five": 1/2=0.5=50%, 1/4=0.25=25%, 1/5=0.2=20%, 1/10=0.1=10%, 3/4=0.75=75%
When converting: always go fraction → decimal → percentage (in that order)
Label your answer: write "25%" not just "25"
Sanity check: a percentage should always be between 0% and 100%

Mistake 4: Wrong Operation in a Worded Problem

❌ The Mistake

Reading "altogether" and adding, when the question asks for "each" (which needs division). Or multiplying when subtracting would be correct.

Example 1:

Question: "24 biscuits are shared equally among 6 children. How many biscuits does each child get?"

Student writes: 24 + 6 = 30 (added instead of divided)

Correct answer: 24 ÷ 6 = 4 biscuits per child

Example 2:

Question: "A book costs £8. If you have £20, how much change do you get?"

Student writes: 20 × 8 = 160 (multiplied instead of subtracted)

Correct answer: 20 − 8 = £12 change

Why Students Make This Mistake:

Worded problems require translating English into maths. Key words like "share", "each", "altogether", "left" signal which operation to use — but students often rush past these clues. They might also panic and just pick an operation without thinking.

Correct Approach — The Operation Keywords:

Addition clues: altogether, total, in total, add, combined
Subtraction clues: left, remaining, change, difference, fewer
Multiplication clues: each, per, times, groups of
Division clues: share equally, per person, each, how many in each group

Practice Tip: Write the operation keywords on a sticky note and keep it near you while practising. When you see a worded problem, scan for these words before you calculate.

How to Avoid This:

Highlight the operation keyword(s) in the question
Write the operation symbol (+ − × ÷) before you calculate
Reread the question and ask: "Am I adding, subtracting, multiplying, or dividing?"
Check: Does the answer make sense? (e.g., "share 24 biscuits among 6 kids" → answer should be much less than 24)

Mistake 5: Forgetting to Include Units in the Answer

❌ The Mistake

Writing "12" instead of "12 cm", or "15" instead of "£15" or "15 minutes".

Example 1:

Question: "The perimeter of a square is 20 cm. What is the side length?"

Student writes: 5

Should be: 5 cm

Example 2:

Question: "A cinema ticket costs £8. How much for 3 tickets?"

Student writes: 24

Should be: £24

Why Students Make This Mistake:

Students focus on the calculation and forget that numbers in maths always have units. They think the unit is "obvious", but examiners mark units as part of the complete answer. In a busy exam, it's easy to skip.

Correct Approach:

Step 1: Identify the unit in the question (cm, metres, £, grams, minutes, etc.).
Step 2: After calculating, write the number AND the unit.
Step 3: Check: Does the unit match what's being asked?

"Perimeter of square = 20 cm. Side = 20 ÷ 4 = 5 cm" ✓

How to Avoid This:

Circle the unit in the question as soon as you read it
Always include the unit when you write your answer
If no unit is given, check whether you should infer one (e.g., if a question asks for "cost", add £)
For time problems, check: is the answer in seconds, minutes, or hours? Write it clearly

Mistake 6: Not Showing Your Working

❌ The Mistake

Writing only the final answer, with no steps shown. Even if the answer is wrong, partial credit is given for correct working.

Example:

Question: "A packet of sweets costs 30p. How much for 5 packets?"

Student writes: 150p

No working shown. If this is wrong, the examiner can't see where the error occurred.

Why Students Make This Mistake:

Students think "showing working" is optional or just takes up time. In a timed exam, they want to answer as fast as possible. But 11+ exams (especially GL Assessment) reward clear working.

Correct Approach:

Always show at least one line of working, even for "simple" problems.

"30p × 5 = 150p" or "30 × 5 = 150, so 150p"

For longer problems, break it into steps:
Step 1: ... = ...
Step 2: ... = ...
Answer: ...

Practice Tip: Practise writing your working more neatly than you think is necessary. A clear, step-by-step solution takes a few extra seconds but saves points if your answer is wrong.

How to Avoid This:

Even for "easy" questions, write one line of working
For multi-step problems, number your steps (Step 1, Step 2, etc.)
Use arrows or equals signs to link each step
Write neatly — if the examiner can't read it, it's marked as wrong

Mistake 7: Misunderstanding Ratio Problems

❌ The Mistake

In a ratio problem, treating the ratio as actual quantities. E.g., ratio 2:3 means 2 items and 3 items, not a total of 5 parts.

Example:

Question: "Sweets are shared in ratio 2:3 between Alice and Bob. Alice gets 2 sweets, Bob gets 3 sweets. How many sweets were shared if the total is 20?"

Wrong approach: "Ratio 2:3 means Alice got 2 and Bob got 3. But wait, that's only 5, not 20. I don't understand this."

Correct approach: "Ratio 2:3 means for every 2 Alice gets, Bob gets 3. That's 5 parts total. If total is 20, then 20 ÷ 5 = 4 per part. Alice: 2 × 4 = 8. Bob: 3 × 4 = 12. Check: 8 + 12 = 20 ✓"

Why Students Make This Mistake:

Ratios are abstract. Students confuse "ratio 2:3" with "2 items and 3 items". They don't understand that a ratio is a pattern that scales up. If you multiply one side by 2, you must multiply the other by 2.

Correct Approach — The Ratio Formula:

Ratio a:b means:
• Total parts = a + b
• Each part = Total ÷ (a + b)
• One side = a × (each part)
• Other side = b × (each part)

Example: Ratio 2:3, total 20
• Parts = 2 + 3 = 5
• Each part = 20 ÷ 5 = 4
• Side A = 2 × 4 = 8
• Side B = 3 × 4 = 12
• Check: 8 + 12 = 20 ✓

How to Avoid This:

Always add up the ratio parts first (2 + 3 = 5)
Find "one part" by dividing the total by the sum (20 ÷ 5 = 4)
Multiply each ratio number by "one part" (2 × 4 = 8, 3 × 4 = 12)
Always check: Do the two sides add up to the total?

Mistake 8: Ordering Decimals Incorrectly

❌ The Mistake

Thinking 0.9 > 0.10, or ordering 0.3, 0.03, 0.30 incorrectly because students compare the digits after the decimal point as if they were whole numbers.

Example 1:

Question: "Order these decimals from smallest to largest: 0.7, 0.07, 0.70"

Student writes: 0.7, 0.70, 0.07 (Wrong!)

Correct order: 0.07, 0.7, 0.70 (Note: 0.7 = 0.70)

Example 2:

Question: "Order these decimals from smallest to largest: 0.9, 0.19, 0.91"

Student writes: 0.19, 0.9, 0.91 (treating it like 19, 9, 91)

Correct order: 0.19, 0.9, 0.91 (actually correct by coincidence, but for the wrong reason!)

Why Students Make This Mistake:

Decimals "look" like whole numbers with a point in the middle. Students compare 0.7 and 0.07 by looking at "7" and "07" and think 07 < 7. They don't think about place value (tenths vs. hundredths).

Correct Approach — Place Value for Decimals:

Think of decimals in terms of place value:
0.7 = 0.70 = 7 tenths = 70 hundredths
0.07 = 7 hundredths
0.70 = 70 hundredths = 7 tenths

Rewrite with the same number of decimal places:
0.7 = 0.70
0.07 = 0.07
0.70 = 0.70

Now it's obvious: 0.07 < 0.70 = 0.70

Practice Tip: Convert all decimals to the same number of decimal places before comparing. It removes ambiguity.

How to Avoid This:

Rewrite all decimals with the same number of decimal places
Think about place value, not just the digits after the decimal
Use a number line: 0 — 0.1 — 0.2 — ... — 0.9 — 1.0
Remember: 0.1 is bigger than 0.01 (just like 1 is bigger than 0.1)

Mistake 9: Incorrect Angle Measurement

❌ The Mistake

Reading a protractor from the wrong scale, or measuring reflex angles (>180°) without realising. Common: writing 120° when it should be 60°.

Example:

Question: "Measure this angle using a protractor."

The angle is 60°, but the student reads from the wrong scale on the protractor and writes 120°.

Why Students Make This Mistake:

Protractors have two scales (inner and outer), and students get confused about which one to use. They might also confuse the position of the angle baseline.

Correct Approach:

Step 1: Place the protractor's centre point on the angle's vertex (corner).
Step 2: Align one arm of the angle with the 0° line.
Step 3: Look at where the other arm crosses the protractor.
Step 4: Check: Does the angle look acute (< 90°), right (= 90°), or obtuse (> 90°)?
Step 5: Read the matching scale (if the angle looks acute, read the scale that gives a number less than 90°).

How to Avoid This:

Always estimate the angle first by eye (acute, right, or obtuse?)
Line up one arm with the 0° baseline before reading
Use your estimate to choose the correct scale (not the wrong one)
If it's a reflex angle (> 180°), measure the smaller angle and subtract from 360°

Mistake 10: Forgetting to Simplify Fractions

❌ The Mistake

Writing 6/8 instead of 3/4, or 10/20 instead of 1/2. The answer is mathematically correct but not in simplest form.

Example:

Question: "Express 15/20 in simplest form."

Student writes: 15/20

Correct answer: 3/4

(Both are equivalent, but 3/4 is the simplified version.)

Why Students Make This Mistake:

Students calculate the fraction correctly but don't know how to simplify it. They're unsure whether simplification is required or don't know the method (find the highest common factor).

Correct Approach — Simplifying Fractions:

Step 1: Find the highest common factor (HCF) of the numerator and denominator.
Step 2: Divide both by the HCF.

Example: Simplify 15/20
• Factors of 15: 1, 3, 5, 15
• Factors of 20: 1, 2, 4, 5, 10, 20
• HCF = 5
• 15 ÷ 5 = 3, 20 ÷ 5 = 4
• Answer: 3/4

Practice Tip: Learn to spot common HCFs: 2, 3, 5, 10. Most 11+ fractions simplify by one of these.

How to Avoid This:

Always check if numerator and denominator can be divided by 2, 3, 5, or 10
If the question says "simplify" or "in simplest form", you must simplify
After simplifying, check: Can it be simplified further? (e.g., 6/9 → 2/3, not 6/9)
Tip: If the numerator and denominator are both even, divide by 2

Mistake 11: Misunderstanding "Mean", "Median", and "Mode"

❌ The Mistake

Confusing the three types of average. Writing the median instead of the mean, or vice versa.

Example:

Question: "Find the mean of 2, 5, 8, 9."

Student looks for the middle value: (5 + 8) ÷ 2 = 6.5 (that's the median!)

Correct answer: (2 + 5 + 8 + 9) ÷ 4 = 24 ÷ 4 = 6 (mean = average)

Why Students Make This Mistake:

Mean, median, and mode are three different statistical concepts taught close together. Students mix them up, especially if they haven't practised distinguishing them.

Correct Approach — The Three Averages:

Mean (average): Add all values, divide by how many values there are.
2 + 5 + 8 + 9 = 24, and there are 4 numbers, so mean = 24 ÷ 4 = 6

Median (middle): Arrange in order, find the middle value.
2, 5, 8, 9 → median = (5 + 8) ÷ 2 = 6.5 (if even number of values, take average of middle two)

Mode (most common): Which value appears most often?
In 2, 5, 8, 9, each appears once, so no mode.

Memory Trick: • Mean = Add all, divide by count (mean is "average")
• Median = Middle value (median sounds like "mid")
• Mode = Most common (mode sounds like "most")

How to Avoid This:

Check the question: does it ask for "mean", "median", or "mode"?
Mean: always add and divide
Median: always sort the values in order first
Mode: count which value appears most often
If unsure, write what each of the three is, then pick the right one

Mistake 12: Incorrect Order of Operations (BODMAS)

❌ The Mistake

Calculating left to right instead of following BODMAS, leading to completely wrong answers on multi-operation problems.

Example 1:

Question: "2 + 3 × 4 = ?"

Wrong approach: 2 + 3 = 5, then 5 × 4 = 20 (left to right)

Correct approach: 3 × 4 = 12 first (multiplication before addition), then 2 + 12 = 14

Example 2:

Question: "10 − 4 ÷ 2 = ?"

Wrong approach: 10 − 4 = 6, then 6 ÷ 2 = 3 (left to right)

Correct approach: 4 ÷ 2 = 2 first (division before subtraction), then 10 − 2 = 8

Why Students Make This Mistake:

Habit from reading left to right makes students calculate left to right. They haven't fully internalised that × and ÷ have higher priority than + and −.

Correct Approach — BODMAS (Order of Operations):

B - Brackets (Parentheses)
O - Orders (Exponents, Powers)
D - Division
M - Multiplication
A - Addition
S - Subtraction

(Note: Division and Multiplication are equal priority — do left to right. Same with Addition and Subtraction.)

Example: 2 + 3 × 4
Step 1: Is there a bracket? No.
Step 2: Is there an order/exponent? No.
Step 3: Is there × or ÷? Yes! 3 × 4 = 12.
Step 4: Now do + or −: 2 + 12 = 14.

Practice Tip: Circle or underline the operations that need to be done first (× and ÷), then do them before + and −.

How to Avoid This:

Underline or circle any × or ÷ — do these first
Then do + or − from left to right
Bracket any × or ÷ operations you've completed
Use the phrase: "Brackets, Orders, Division/Multiplication, Addition/Subtraction"

Summary: The Top 5 Habits to Prevent Mistakes

Impact of Habit Training on Preventing Mistakes

By implementing these 5 habits in just 50 seconds per question, you can prevent most common mistakes and improve your score by 10–15%.

Habit	What It Prevents	How Long It Takes
Read the question twice	Missing key information; forgetting to answer all parts	10 seconds
Show your working	Careless arithmetic errors; losing partial credit	20 seconds
Write units with your answer	Forgetting metres, pounds, minutes, etc.	5 seconds
Sanity check: Is the answer reasonable?	Absurd answers (e.g., 1000 biscuits per child)	10 seconds
Underline operation keywords	Wrong operation (× instead of ÷)	5 seconds

Total extra time: 50 seconds per question — which catches most major errors.

How to Practice to Avoid These Mistakes

1. Take Practice Tests Under Timed Conditions
You can't fix speed errors if you only practise slowly. Time yourself to match the real exam.

2. Review Every Wrong Answer
Don't just look at the correct answer. Ask: "Which of the 12 mistakes did I make?" Then practice that specific mistake type.

3. Keep an Error Log
Write down each mistake type you make (e.g., "Forgot units on 3 geometry questions"). This shows which mistakes are personal habits for you.

4. Practice the Mistake Intentionally
If you repeatedly forget units, do 10 questions focusing only on writing units. Boring, but effective.

5. Slow Down in the Final 5 Minutes
Many mistakes are pure speed errors. In the last 5 minutes of the exam, sacrifice a few marks on harder questions to double-check the easier ones you've already answered.

Practice 11+ Maths with Detailed Feedback

Further Resources

Khan Academy: Free videos on all 11+ Maths topics
BBC Bitesize: Structured lessons by topic for KS2/3
Past Papers: Download past 11+ papers from your target school's website
Homework Magic: Timed practice papers with detailed error breakdown

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