Detective maths quest: decimals
A story-led junior maths adventure using evidence, charts and mystery patterns to measure small distances, money and scores with tenths and hundredths.
Quick answer
Decimals are a way of writing numbers that are smaller than one, using tenths and hundredths. You can think of them like parts of a whole, and they help you measure, count, and share things more precisely.
Learning outcome
By the end of this page, you will be able to read, write, and use decimals to show parts of a whole in real-life situations like measuring distance, counting money, or scoring in games.
- You can show a decimal number like 0.3 or 0.75 using pictures or objects
- You can compare two decimal numbers to see which is bigger or smaller
- You can use decimals to measure small amounts in everyday life
- You can solve simple problems using decimals in games, shopping, or sports
Why this matters
Decimals help us be more precise when we measure or count things that are not whole numbers. For example, if you're sharing a pizza and you eat 0.5 of it, that means half. If you're timing a race and it takes 12.3 seconds, that’s 12 seconds and 3 tenths of a second. Decimals are used in many places like shopping, cooking, sports, and even in video games. Learning decimals helps you understand how the world works in a more accurate way.
How it works
Decimals are numbers that have a decimal point (a dot) to show parts of a whole. The numbers to the right of the decimal point are smaller parts:
- The first number after the decimal point is called the tenth (1/10)
- The second number after the decimal point is called the hundredth (1/100)
Think of a whole pizza cut into 10 equal slices. If you eat 3 slices, you have eaten 3 tenths of the pizza, or 0.3 of it. If you cut each slice into 10 smaller pieces, and eat 3 of those, you have eaten 3 hundredths, or 0.03 of the pizza.
Examples you can see
Let’s imagine you are a detective solving a mystery. You find a clue that says "The treasure is hidden 0.5 metres from the tree." This means the treasure is halfway between the tree and the next point. Let’s look at how decimals are used in different situations:
| Real-life situation | Decimal number | What it means |
|---|---|---|
| Measuring a piece of string | 0.7 m | Seven tenths of a metre |
| Counting money | £2.45 | Two pounds and forty-five hundredths of a pound |
| Scoring in a game | 10.8 points | Ten points and eight tenths of a point |
| Time | 3.25 seconds | Three seconds and twenty-five hundredths of a second |
Try it as a mini adventure
Imagine you are a detective and you have found a mysterious note that says:
"The secret map is hidden under 0.6 of the old oak tree's roots."
Your mission is to find the exact spot! You can use a ruler or a measuring tape to mark 0.6 of a metre from the tree. If the tree is 1 metre tall, you need to find the spot that is 6 tenths of the way up the tree trunk. Or if you're walking and you have walked 0.3 of a kilometre, how far is that? You can practice by walking 3 tenths of a metre or 30 centimetres.
Mini Challenge:
- Draw a line that is 0.4 metres long.
- Measure 0.8 of a 100-metre track.
- Find 0.25 of a 100-piece puzzle.
Common mistakes
- Mistake: Thinking 0.5 is the same as 0.05
Why it happens: Confusing the place value of tenths and hundredths
Fix: Use a visual like a grid to show that 0.5 is 50 out of 100, while 0.05 is only 5 out of 100 - Mistake: Adding decimals like 0.3 + 0.2 = 0.53
Why it happens: Adding the digits but not understanding place value
Fix: Align decimals by the decimal point and add tenths with tenths - Mistake: Writing 0.3 as 3 tenths and 0.03 as 3 hundredths
Why it happens: Mixing up how many digits are in the decimal
Fix: Remember: 0.3 = 3 tenths, 0.03 = 3 hundredths
Parent and carer guide
Helping your child understand decimals can be fun and practical:
- Use everyday situations like cooking, shopping, or measuring to talk about decimals.
- Ask your child to estimate how much 0.5 of a pizza is, or how many 0.25 of a cake is.
- Use a visual like a grid or a pie chart to show what 0.5 and 0.05 look like.
- Play a game where you guess how many tenths or hundredths of a metre a step is.
- Don’t worry if they make mistakes—decimals take time to understand. Be patient and encouraging!
Practice questions
- What does 0.4 mean?
Answer: Four tenths - Which is bigger: 0.7 or 0.07?
Answer: 0.7 - How much is 0.3 of a 100-piece puzzle?
Answer: 30 pieces - Write 0.6 as a fraction.
Answer: 6/10 - What is 0.2 + 0.3?
Answer: 0.5 - What is 0.5 of a 100-metre race?
Answer: 50 metres - How many hundredths are in 0.8?
Answer: 80 hundredths - Which decimal is the same as 0.25?
Answer: 25 hundredths
FAQ
What is a decimal?
A decimal is a way of writing a number that is smaller than one, using tenths and hundredths. For example, 0.5 means five tenths, and 0.25 means twenty-five hundredths.
Why do we use decimals?
Decimals help us be more precise when we measure or count things that are not whole numbers. For example, if you're measuring how tall you are or how much money you have, decimals make it easier to be exact.
How are decimals used in games?
In games, decimals are used for scores, timing, and measurements. For example, a race might take 12.3 seconds, or a character might move 0.5 units to the right.
How can I practice decimals at home?
You can practice decimals by measuring things around your house, counting money, or using a ruler to find 0.5 of a metre. Try drawing a line that is 0.3 metres long or dividing a pizza into 10 equal slices.
What is the difference between 0.5 and 0.05?
0.5 is five tenths, which is much bigger than 0.05, which is only five hundredths. You can see this by drawing a grid and shading 50 squares for 0.5 and only 5 squares for 0.05.