Submarine maths quest: decimals

Quick answer

You can use decimals to measure small distances, count money, and score points in games when you need to work with parts of a whole number. In this submarine adventure, you'll use decimals to explore depths, pressure levels, and hidden coordinates to find treasure.

Learning outcome

By the end of this page, you will be able to use decimals to measure small distances, count money, and score points in games, and understand how tenths and hundredths work in real-life situations.

Success signs:

• You can read and write decimals like 0.5, 1.25, and 3.7
• You can compare decimals to see which is bigger or smaller
• You can add and subtract decimals in simple situations
• You can use decimals to solve real-life problems like measuring depth or counting money

Why this matters

Decimals help us measure things that are not whole numbers, like how deep the ocean is, how much money we have, or how far we've swum. When you play games, watch sports, or go shopping, you'll see decimals everywhere. Understanding decimals helps you become a better problem solver and makes you more confident with numbers.

How it works

Decimals are numbers that have a decimal point (like 3.45). The digits after the decimal point show parts of a whole number:

1. The first digit after the decimal point is the tenths place (1/10)
2. The second digit after the decimal point is the hundredths place (1/100)
3. The third digit is the thousandths place (1/1000)

Think of it like money:

• £1.50 means 1 pound and 50 pence
• £0.25 means 25 pence (a quarter)

Examples you can see

Let's go on a submarine adventure! Your submarine is exploring the deep ocean. Here are some examples of how decimals help you in your journey:

Depth measurements

Your submarine dives down to different depths:

• 12.5 metres deep
• 8.25 metres deep
• 25.75 metres deep

You can see that 25.75 is deeper than 12.5 because 25 is bigger than 12.

Pressure readings

As you go deeper, the pressure increases:

• At 10.25 metres, pressure is 10.25 atmospheres
• At 15.75 metres, pressure is 15.75 atmospheres

Notice how 15.75 is bigger than 10.25 because 5 tenths (0.75) is more than 2 tenths (0.25).

Hidden coordinates

Find the treasure by reading coordinates:

• Find treasure at 3.5, 7.25
• Find treasure at 1.75, 4.5

The first number is the horizontal position, and the second number is the vertical position.

Money in the treasure chest

You find a treasure chest with coins:

• 2.50 pounds in coins
• 1.25 pounds in coins

When you add them together: 2.50 + 1.25 = 3.75 pounds

Try it as a mini adventure

Imagine you're a submarine explorer! You have a mission to find treasure at specific coordinates. Your submarine's computer shows these depths:

1. First stop: 15.25 metres
2. Second stop: 22.75 metres
3. Third stop: 18.5 metres
4. Fourth stop: 27.3 metres

Let's find the treasure by solving these challenges:

1. What is the deepest stop? Answer: 27.3 metres
2. What is the shallowest stop? Answer: 15.25 metres
3. How much deeper is the fourth stop than the first? Answer: 12.05 metres
4. What is the total depth of all stops? Answer: 83.8 metres

Common mistakes

• Mistake: Thinking 0.8 is bigger than 0.25 because 8 is bigger than 2
  Why it happens: Confusing the decimal point position
  Fix: Remember that 0.8 = 0.80, so 0.80 is bigger than 0.25
• Mistake: Adding decimals by just adding the numbers without considering decimal places
  Why it happens: Not lining up decimal points
  Fix: Write numbers in columns with decimal points aligned
• Mistake: Forgetting that 0.1 = 1/10 and 0.01 = 1/100
  Why it happens: Not connecting decimals to fractions
  Fix: Use visual models like grids or money to show the relationship

Parent and carer guide

Help your child practice decimals by:

1. Playing with money at home - count out 1.25 pounds or 2.75 pounds
2. Measuring distances around the house using metres and centimetres (like 1.5 metres)
3. Looking at clocks and times with decimals (like 2.5 hours)
4. Using cooking recipes that involve half portions (like 0.5 of a cup)
5. Playing games that involve scoring with decimals (like 12.7 points)

Don't worry if your child makes mistakes - they're learning! Focus on understanding the concept, not just getting the right answer.

Practice questions

1. Which is bigger: 0.4 or 0.35? Answer: 0.4
2. What is 0.6 + 0.25? Answer: 0.85
3. What is 1.75 - 0.5? Answer: 1.25
4. Write 0.75 as a fraction. Answer: 75/100 or 3/4
5. Which decimal is between 0.2 and 0.3? Answer: 0.25
6. What is 2.5 + 1.75? Answer: 4.25
7. What is 3.2 - 0.8? Answer: 2.4
8. How many tenths are in 0.6? Answer: 6 tenths
9. How many hundredths are in 0.85? Answer: 85 hundredths
10. Which is smaller: 0.9 or 0.95? Answer: 0.9

FAQ

What is a decimal?

A decimal is a number that has a decimal point. It helps us write parts of a whole number, like 0.5 (half) or 0.25 (a quarter).

How do I read decimals?

Read decimals by saying the number followed by the place value. For example, 0.75 is "seventy-five hundredths."

Where do I see decimals in real life?

You see decimals when measuring things like height (1.65 metres), money (£2.50), time (2.5 hours), or cooking (0.5 cup of sugar).

How do I add decimals?

Line up the decimal points and add like normal numbers. For example, 1.25 + 0.75 = 2.00.

Why are decimals important?

Decimals help us measure things more precisely. They're used in science, cooking, money, maps, and many other everyday situations.