[PDF] Adding and Subtracting Decimals

2586_6year_5_adding_and_subtracting_decimals_guidance_page.pdf

Adding and Subtracting Decimals

Over the last few weeks you have been re-visiting decimal numbers. Hopefully you are now confident with the

place value of decimal numbers and how to compare and order them. We will now start looking at how to

solve problems with decimal numbers. Remember, you can go back and re-visit any previous tasks if you need a

reminder.

Decimal Compliments

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0Ck BAABC 0Ą AE7C 3E 7 0ĄA0AB 0Ą EBA70A ONE WHOLE. When

looking at decimal compliments, it is helpful to think back to the number bonds to 10 and 100 which you are

confident with. This part-part whole model is asking me to work out what I would need to add to 0.4 to make 1.

I know that 4 + 6 = 10 and 40 + 60 = 100

Therefore, I think that 0.4 + 0.6 = 1

I could use a bar model to check my answer

0.4 0.6

Adding Decimals

When we add decimals we use many of the same rules that we use to add whole numbers. The important thing to remember when calculating with decimal numbers is that we need to take care over the place value of numbers.

Example 1:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Example 2:

Place value grids can be helpful when adding decimal numbers. I can see here that the grid represents the number 0.412

What number is one hundredth more?

If I were to place another hundredths column I would have 2 hundredths (0.02) and this would be the only column to change.

My answer would be 0.422

We can also use column addition to add decimal

numbers. How we set out our calculation is really important. The decimal points must line up to ensure that our numbers keep the same place value. The decimal point should also line up in our answer. We then work from right to left, exchanging where needed.

The answer should be 7.59

Subtracting Decimals

When we subtract decimals we use many of the same rules that we use to subtract whole numbers. Like

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30 7 ; 0A ĄB07 0 0  0Ą BAA8AB 3A E7E170  30 AE7

18ABC C

that we need to take care over the place value of numbers. Sometimes it is helpful to use rounding to

make a prediction of what your answer should be to help check the accuracy of your answer.

Example 1:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Example 2:

I can see that the number shown here is 0.344

Take away 0.2, what is your number now?

If I took away 2 of the tenths I would have one tenth left (0.1) and the rest of the columns would stay the same.

My answer would be 0.144

We can also use column subtraction with decimal

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A 3A 3A 7A 0A; 0kC BA7D important that we line up our decimal points correctly. Occasionally we have to exchange numbers and this is done in the same way as when we subtract with whole numbers.

Starting from the right, I cannot do 3  4

I can exchange a tenth from the left to give me 10 hundredths. Then I can calculate 13- 4 = 9

2  1 = 1

4  2 = 2

So my answer would be 2. 19

Try these questions to practice the method:

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18ABC 0Ą 

A 1K

Sometimes your decimal numbers will have a different number of digits, which can look confusing when you start to calculate with them. For example, in the problem below, the first number does not have any hundredths but the second number does. We cannot just forget about these hundredths! Instead, we could use a 0 as a place holder to help us line everything up correctly. place holder

Can you find the answer to the calculation?

Useful Videos

These videos may help you understand

the methods we can use to add and subtract decimal

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k0 ĄBA0

0ABA C 7CĄ 0A jk 8100Ą

70 0A 0Ą Ą
any Mathletics task to help you with each task.

Adding and subtracting decimals using column

method: https://www.youtube.com/watch?v=gD- h0HGurg8

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A 1k DĄ1B

decimal numbers: https://www.youtube.com/watch?v=dlotpOS8NFA

Adding and Subtracting Decimals - Challenges

We know that lots of you like to take on a challenge during our maths lessons. These challenges are not

compulsory, but are there for anyone who wishes to give them a go. Good luck!

All of these challenges were taken from the

White Rose Maths Hub. They have also produced

7 jGĄA A7B

 k CAE0Ą 0Ą 0AB 3A8C0AK - here are videos which may help you with your learning. https://whiterosemaths.com/homelearning/year-5/