Common Primary 5 Math Mistakes in Singapore and How to Fix Them
October 25, 2026
This guide covers the error types that show up in Primary 5 (P5) math work in Singapore. For each one, it shows what the mistake looks like, which P5 topic it belongs to where there is one, and one check and one practice fix you can try at home. The fixes are teaching-practice suggestions, not measured results. In the Ministry of Education (MOE) syllabus, ratio, average, and percentage increase and decrease are Primary 6 (P6) topics, so this guide does not give fixes for them. Speed does not appear in the P5 topic list of that syllabus, so the guide gives no fixes for it. To use it, run the four-step check in the next section on any wrong answer, then read the section for the error you see.
How to check a wrong answer
When a worked answer is wrong, look at four things in this order.
- The question. Read it twice. Underline the key words, such as “remaining”, “each” or “altogether”, and say the question in your own words. What is it actually asking for?
- The whole. The whole is the full amount that a fraction or percentage is taken from. Before calculating, write down what the whole is, then check whether your child used that one.
- The units. Are all the numbers in the same unit? Metres and centimetres, or dollars and cents, must be converted to one unit before they are added or subtracted.
- The place value. Place value is the worth of a digit based on where it sits. In 3.25, the 2 means 2 tenths. Check that the decimal point is where it should be, and ask whether the answer is a sensible size.
Working through these four points in order helps you find where a slip happened, even when the arithmetic itself looks fine.
Missing key words in word problems
A word problem is a short story with a question at the end. A child who reads the numbers and starts calculating straight away can skip the word that changes what has to be worked out. A word such as “remaining” means something has already been taken away, so a child who skips it may add when they should subtract.
This is a reading skill rather than a P5 topic, so it applies to every topic in this guide.
Check: Ask your child to say the question back in their own words before writing any numbers.
Practice fix (teaching-practice suggestion): Give each word problem a quick first pass. Underline the key words, restate the question in one sentence, and only then start calculating.
Taking a fraction of the wrong quantity
P5 fraction work includes dividing a whole number so that the answer is a fraction, working with mixed numbers (a whole number and a fraction, such as 2 and 1/4), and multiplying fractions. One error to watch for is taking a fraction of the original amount when the question asks for a fraction of what is left.
Hypothetical example, with made-up numbers: a jar has 60 marbles. Mei gives away one third of them, then gives away half of the marbles that are left. How many does she have now? She gives away 20, leaving 40. She then gives away half of those 40, which is another 20, so she has 20 left. The wrong method takes half of the original 60 (30) from the 40 left, which leaves 10.
Check: Before calculating, ask “a fraction of what?” and name the whole.
Practice fix (teaching-practice suggestion): Draw a bar model. A bar model is a simple drawing made of rectangles, where each rectangle shows a quantity so the parts and the whole are visible. For a before-and-after problem, draw one bar for the start, then a second bar for what remains after each step. Each fraction is then taken from the bar it belongs to.
Unit slips in conversions and volume
Converting a measurement from a smaller unit to a larger one, or the reverse, is part of P5 decimal work. Examples are kilometres and metres, metres and centimetres, kilograms and grams, and litres and millilitres. Volume is measured in cubic units, such as cubic centimetres (cm³). A cubic unit is a small cube. So cm³ measures the space inside a solid, while cm measures length. The P5 syllabus links litres and millilitres to cm³, but it excludes converting between cm³ and m³, so this guide does not practise that conversion.
Hypothetical example, with made-up numbers: a rope is 1.5 m long and a second rope is 45 cm long. What is their total length? Adding 1.5 and 45 gives 46.5, which mixes metres with centimetres. Converting 1.5 m to 150 cm first gives 150 + 45 = 195 cm.
Check: Ask “what unit is this number in?” for every number in the working.
Practice fix (teaching-practice suggestion): Write the unit on every line of working. Convert everything to one unit before calculating. Remember that cm and cm³ are different measures, so a length cannot be added to a volume.
Percentage of the wrong base
P5 percentage work covers expressing a part of a whole as a percentage, finding a percentage of a whole, and calculations involving discount, GST (Goods and Services Tax, a tax charged on goods and services in Singapore) and annual interest (the yearly amount charged or earned on a sum of money). A percentage is a number of parts out of 100. The error to watch for is using the wrong base, which means the amount the percentage is taken from.
Hypothetical example, with made-up numbers: a bag costs $80 and is on sale at 25% off. The discount is 25% of $80, which is $20, so the sale price is $60. If the shop then takes a further 25% off the sale price, the second discount is 25% of $60, which is $15, so the final price is $45. The wrong method works out the second 25% on the original $80, which is $20, and takes that off $60, leaving $40.
Check: Ask “percentage of what?” before calculating.
Practice fix (teaching-practice suggestion): Write the base in words, such as “25% of $80”, then calculate. If a question mentions a price before and after a change, mark which of the two amounts the percentage applies to.
Decimal place-value slips when multiplying or dividing by 10, 100 or 1000
P5 decimal work includes multiplying and dividing decimals (numbers with a decimal point, up to 3 decimal places) by 10, 100, 1000 and their multiples, without a calculator. The slip can look like a decimal point that has moved the wrong way. For example, 3.25 multiplied by 100 is 325, so writing 0.325 means the point moved the wrong way. Dividing 3.25 by 10 gives 0.325, so writing 32.5 means the point moved the wrong way.
Check: Estimate first. Since 3.25 is a little more than 3, 3.25 multiplied by 100 should be a little more than 300. Dividing it by 10 should give something less than 1.
Practice fix (teaching-practice suggestion): Before calculating, estimate and say whether the answer should be bigger or smaller than the starting number. Then check the direction the decimal point moves. Multiplying by 10, 100 or 1000 makes the number bigger. Dividing by them makes it smaller.
Copying errors between steps
A number can be copied wrongly from one line of working to the next. The rest of the working can look sound even though the answer is wrong, which makes it hard to tell where to look. This is a teaching-practice observation, not a measured pattern.
Check: Compare each number in the final line with the number it came from, one at a time.
Practice fix (teaching-practice suggestion): Point to each number with a finger or pencil as you copy it onto the next line.
Rushing and test anxiety
Some children rush through questions they could answer, skip the checking step, or freeze when there is a timer. This is also a teaching-practice observation. This guide does not claim that any routine fixes it.
Practice fix (teaching-practice suggestion): Do a short set of questions with a timer, then go through the same questions untimed and explain each step out loud. The untimed round can help you tell whether the problem was rushing or understanding.
What is outside P5
This guide does not cover ratio, average, percentage increase and decrease, or speed. In the MOE Mathematics Syllabus, Primary One to Six (the 2021 Primary Mathematics Syllabus, updated October 2025), ratio, average, and percentage increase and decrease are listed under P6. Speed does not appear in the P5 topic list of that syllabus, so the guide gives no fixes for it. Some tuition guides list ratio as a P5 topic. Check the syllabus before following that advice.
The guide also does not cover the format of the Primary School Leaving Examination (PSLE), the national exam taken at the end of primary school, and it makes no claims about exam changes.
Your next step
Run this checklist on any wrong answer:
- Read the question twice and say it in your own words.
- Name the whole before calculating.
- Write the unit on every line, and convert to one unit before you calculate.
- Estimate first, then check where the decimal point sits.
If you want structured practice on these topics, P5 math tuition is one option to compare with others.
