Year 6 Maths Addition and Subtraction Study flashcards

[q] Tricks for Adding and Subtracting

Sometimes you can use tricks to make your calculations easier. This is called calculating and adjusting.

[a] Example 1
23 + 9
If you think of the 9 as a 10 (9 + 1), it’s easier to add:
23 + 10 = 33
But remember you added 10 instead of 9, so you must subtract 1 from the answer:
33 − 1 = 32
23 + 9 = 32

Example 2
23 + 11
In the same way, you can think of 11 as a 10 (11 − 1):
23 + 10 = 33
But remember you added 10 instead of 11, so you must add 1 more to the answer:
33 + 1 = 34 23 + 11 = 34

This trick works for subtraction too!

Example
42 − 9
Think of the 9 as a 10:
42 − 10 = 32
But remember you took away 10 instead of 9, so you must add 1 to the answer:
32 + 1 = 33 42 − 9 = 33

You can use this trick to add bigger numbers.

Example
50 + 199
Add 1 to make 199 into 200 (199 + 1).
Calculate: 50 + 200 = 250
Then adjust: 250 − 1 = 249
50 + 199 = 249

[q] Adding and Subtracting Multiples of 10

You can simplify addition calculations involving multiples of 10.

[a] Example

7 + 15
You can easily calculate 7 + 15 = 22

This method of simplifying works for subtraction too!
140 − 90
14 − 9 = 5
So 140 − 90 = 50

[q] Estimating Answers

It helps to estimate what the answer might be before you start calculating. Then you can check your answer against your estimate to see if it’s correct. You estimate by rounding the numbers.

[a] Example

Then calculate mentally: 20 + 8 + 40 + 1 = 69 and check it against the estimate.
The answer is close to the estimate, so you know you must be correct!

[q] Mental Addition

To add numbers mentally (in your head), it can help to partition them into hundreds, tens and units.

[a] 

You can put the numbers into an order which makes them easier to add up:
200 + 100 + 60 + 40 + 7 + 5
300 + 100 + 12 = 412

[q] Mental Subtraction

You can use partitioning to subtract numbers too.

[a] Example
68 − 43
= 60 + 8 − 40 − 3
= 60 − 40 + 8 − 3
= 20 + 5 = 25
68 − 43 = 25

[q] Addition Using the Column Method

If you are given a sum and the numbers are too big or there are too many numbers to add mentally, then you can use a written method.
You can use the column method to add numbers.

[a] Example 1
Jo has collected 243 football cards and Zara has collected 142 cards. How many cards do the children have altogether?

1. Start with the least significant digit so add the units first.

2. Then add the tens.

3. Finally, add the hundreds.

Example 2
Ahmed has 346 stamps in his collection. His friend Sam has 267 stamps in his collection. How many stamps do the boys have in total?

Record the 3 in the units column and carry the 10 as a 1 in the tens column:

Record the 11 as 1 in the tens column and carry the 10 as a 1 into the hundreds column.

[q] Adding Decimals

Some numbers contain a decimal point, for example 13.51 You can use the column method to add decimals.

[a] Saira has saved £34.62 in pocket money. Her auntie gives her another £23.65. How much money does Saira have now?

First bring the decimal point down and put it in the answer line directly below the decimal points that are already there.

Then add the digits using the column method.

[q] Subtraction Using the Column Method

You can subtract bigger numbers using the column method. You need to make sure that the digits are all written in the correct column.
Sometimes it helps to put the place value labels above your calculation.
Start with the least significant digit (in this calculation – the units).

[a] Example
1. Subtract the units: 5 − 2 = 3

2. Then subtract the tens: 6 − 4 = 2

3. Then subtract the hundreds:
8 − 3 = 5

4. Then, finally, the thousands:
4 − 1 = 3

Some calculations can be more tricky:

Example

1. When you look at the units you can’t subtract 6 from 3, so you go to the tens column and exchange the 7 for a 6 and a 1.

2. Now you have 13 − 6 which you can subtract.

3. You then subtract 6 − 4 in the tens column.

4. When you look at the hundreds column, you can’t subtract 3 from 2 so you go to the thousands column and exchange the 5 for a 4 and a 1. This means you can subtract 12 − 3 = 9.

5. Then you can finish by subtracting 4 − 1 in the thousands column.

[q] Subtracting Decimals

You can subtract decimals using the same method you used for adding decimals.

[a] Example: You can’t subtract 7 from 3 so you need to exchange.