Mermaid maths quest: decimals

Quick answer

Decimals are numbers that are smaller than one whole, shown with a decimal point. You can use decimals to measure small distances, count money, or score points in games – like when a mermaid counts her pearl shells or measures how far she swims.

Learning outcome

By the end of this page, you will understand how to use decimals to show parts of a whole, and how to count, compare, and use tenths and hundredths in real-life situations – just like a mermaid would use her shells and pearls!

• You can explain what a decimal is using objects like shells or coins
• You can compare decimals like 0.5 and 0.75 using a number line or visual model
• You can use decimals to measure small distances or count small amounts of money
• You can play a game where you match shells to decimal numbers

Why this matters

Decimals are like tiny pieces of a whole – they help you count and measure things that are not quite a whole number. For example, a mermaid might swim 2.5 metres or collect 0.75 of a pearl. In real life, decimals help us count money, measure ingredients in recipes, and even keep score in games. Understanding decimals helps you become a better problem solver!

How it works

Decimals are numbers that have a decimal point (like 0.5 or 1.25). The numbers after the decimal point show parts of a whole. For example:

1. 0.1 means 1 part out of 10 equal parts
2. 0.01 means 1 part out of 100 equal parts
3. 0.5 means 5 parts out of 10 equal parts

You can think of decimals like a fraction but written differently. For example:

• 0.5 = 5/10 = half
• 0.25 = 25/100 = a quarter
• 0.1 = 1/10 = one-tenth

Examples you can see

Let’s imagine a mermaid named Marina who collects shells. Each shell is worth a small amount of money or a small part of a pearl. Here are some examples:

Marina swims 0.5 km in the morning and 0.25 km in the afternoon. How far did she swim in total?

0.5 + 0.25 = 0.75 km

Try it as a mini adventure

Mermaid's Pearl Quest

Marina is on a quest to collect shells and pearls. She needs to count how many shells she has and how much money they are worth. Here's your challenge:

1. Marina finds 3 shells that are worth 0.1 each. How much are they worth in total?
2. She finds 2 shells worth 0.25 each. What is the total value?
3. She finds 1 shell worth 0.75 and 1 shell worth 0.25. How much are they worth together?
4. She wants to buy a pearl that costs 0.90. If she has 0.50, how much more does she need?

Adventure tip: Draw a number line to help you add the decimals!

Common mistakes

• Mistake: Thinking 0.5 is the same as 0.05
• Why it happens: The decimal point is easy to miss or confuse with the number of digits
• Fix: Always look at the place value – 0.5 is five tenths, 0.05 is five hundredths

• Mistake: Adding 0.5 + 0.3 = 0.8 instead of 0.80
• Why it happens: Children may not understand that 0.5 and 0.50 are equal
• Fix: Use visual models like shells or coins to show that 0.5 = 0.50

• Mistake: Writing 0.10 instead of 0.1
• Why it happens: Some children think adding a zero makes a number bigger
• Fix: Remind them that 0.1 and 0.10 are the same amount

Parent and carer guide

Help your child understand decimals by using real-life examples like money or measuring ingredients. For example:

• Count out 0.25 of a pound in coins
• Measure how much water they drink in litres using a decimal scale
• Use a number line to show 0.5 and 0.75

Try to make it fun! Ask your child to imagine they are a mermaid collecting shells and counting their treasure. This makes the learning more engaging and memorable.

Don’t worry: It’s normal for decimals to feel tricky at first. Keep practicing with small steps and real-life examples. Encourage your child to draw models or use physical objects like shells or coins to help them see the decimal parts.

Practice questions

1. What is 0.1 + 0.2?
2. How much is 0.50 + 0.25?
3. Which is larger: 0.4 or 0.35?
4. What is 0.75 – 0.25?
5. How much is 3 shells worth if each is worth 0.10?

Answers:

1. 0.3
2. 0.75
3. 0.4
4. 0.50
5. 0.30

FAQ

What is a decimal?

A decimal is a number that shows a part of a whole. It uses a decimal point to separate the whole number from the parts. For example, 0.5 means 5 parts out of 10.

How do I add decimals?

Line up the decimal points and add as usual. For example, 0.3 + 0.4 = 0.7.

Why are decimals important?

Decimals help us measure small amounts, count money, and keep score in games. They are used in science, cooking, and shopping.

Can decimals be more than 1?

Yes! Decimals can be more than 1, like 1.5 or 2.75. These are whole numbers with parts added on.

How can I practice decimals at home?

Use coins, shells, or measuring cups to practice counting and adding decimals. Try games or activities where you measure things and write the amounts as decimals.