Same form, then compare: fractions
The Short Answer
Fractions, decimals and percentages are three ways of writing the same thing, so the first move in almost every question is to put everything into the same form and only then compare. Per cent means out of a hundred, and once that is understood every conversion follows from it.
What are these questions about?
Fractions, decimals and percentages are the same idea wearing three different coats.
That sentence does most of the work in this topic. Once a child sees it, converting stops being three separate skills and becomes one move made in different directions.
Start with per cent
Per cent means out of a hundred. So 40% is simply 40 hundredths — 40/100.
Once that is seen, every conversion follows from it:
- Decimal to percentage: multiply by a hundred
- Percentage to decimal: divide by a hundred
- Fraction to decimal: make the bottom into ten, a hundred or a thousand, then read it off
- Fraction to percentage: go through the decimal, or make the bottom into a hundred directly
Same form, then compare
This is the rule for the whole topic.
Is 2/5 bigger than 0.35? Nobody can tell by looking. Convert: 2/5 is 0.4, and 0.4 is bigger than 0.35.
Order 3/8, 0.4 and 35%. Convert all three to decimals — 0.375, 0.4, 0.35 — and the order falls out.
Can you show me a worked example?
Adding fractions
The bottoms must match before you add, because the bottom number says what size the pieces are.
2/3 + 1/6. Thirds and sixths are different sizes, so convert: 2/3 is 4/6. Now 4/6 + 1/6 = 5/6.
Adding them without converting is the same error as lining decimals up by their ends — putting things of different sizes in the same column.
Where do pupils lose marks?
Comparing across forms without converting, and guessing.
Adding fractions with different bottoms, which produces an answer that looks reasonable.
Multiplying by a decimal under pressure when building from 10% would have been safer.
Forgetting to simplify when the answer is being written down.
Try it yourself
Put everything into the same form before comparing or calculating. Say which form you are converting to before you start.
- Write 3/8 as a decimal.
- Write 0.375 as a percentage.
- Simplify 18/24.
- What is 2/3 + 1/6?
- What is 35% of 80?
- 0.375. Make the bottom into a thousand: 3/8 is 375/1000, which reads straight off as 0.375.
- 37.5%. A decimal becomes a percentage by multiplying by a hundred, and the point moves accordingly — or rather, the digits do.
- 3/4. Divide top and bottom by 6. Simplifying is not decoration: comparing 18/24 with 5/8 is hard, and comparing 3/4 with 5/8 is much easier.
- 5/6. The bottoms must match before adding, so 2/3 becomes 4/6, and 4/6 + 1/6 is 5/6. This is exactly the same idea as padding decimals before adding them.
- 28. Ten per cent of 80 is 8, so 30% is 24; 5% is half of 10%, which is 4; and 24 + 4 is 28. Building from 10% is quicker and safer than multiplying by 0.35 under pressure.
How much practice does this type need at home?
Little and often, and one exercise beats all others: say a number in all three forms.
Half. 0.5. Fifty per cent. A quarter. 0.25. Twenty-five per cent. Three eighths, 0.375, 37.5%.
Do it in the car with the easy ones until they are instant, then add the awkward ones — a third, an eighth, two fifths. A child who holds all three forms of the common fractions in their head has removed most of the work from this topic before the question is even read.
Free: a Maths workbook
Questions across the Maths question types, with the method printed at the top and full answers at the back. One workbook for the whole strand, not a sheet per question type.
Frequently Asked Questions
Why does converting to the same form matter so much?
Because comparing across forms is where children guess. Is 2/5 bigger than 0.35? Nobody knows by looking. Turn 2/5 into 0.4 and it is obvious. The rule we teach is 'same form, then compare' and it applies to ordering, to adding, and to any question offering a mixture of fractions, decimals and percentages among its options.
What is the quickest way to find a percentage of an amount?
Build from ten per cent. Ten per cent of 80 is 8, so 20% is 16, 30% is 24, and 5% is half of 10%, which is 4. Almost every percentage in an 11+ question can be assembled from tens and fives and halves, and it is far more reliable under pressure than converting to a decimal and multiplying.
Should my child always simplify a fraction?
Simplify when it helps and do not agonise when it does not. It helps enormously for comparing — 3/4 against 5/8 is manageable, 18/24 against 5/8 is not. It matters less in an intermediate step. Where an answer is being written down, simplified is usually what is wanted, so it is a sensible habit to finish with.
Why must the bottoms match before adding?
Because the bottom number says what size the pieces are. Two thirds and one sixth are pieces of different sizes, and adding them directly is like adding tenths to hundredths — the same error as lining decimals up by their ends. Making the bottoms match is the fraction version of padding, and pointing that out helps children see the two topics as one idea rather than two rules.
Sources
- GL Assessment — 11+ familiarisation materials and question type descriptions Published 2025.
- Clever Clogs Academy — our own Year 4 and Year 5 lesson records, Winslow Published September 2025 to July 2026.
We are not affiliated with, endorsed by or connected to GL Assessment or Buckinghamshire Council. Test arrangements change; always check the council's own pages for the current year's dates.
We Teach This Method Every Week in Winslow
Groups of no more than six pupils with two qualified teachers, in person or online. Come and see a session before you decide.