Use Suitable Methods to Add and Subtract Numbers Mentally

We are going to look at solving addition and subtraction mentally.

This means that we need to find the answers using just our brains!

Luckily, there are lots of different strategies we can use to help us.

 

Let's have a look at some of these strategies.

 

Near Doubles

If we are given an addition question where the numbers are nearly the same, we can use our knowledge of doubling to help.

61 + 60 =

We can double 60 and then add 1

Double 60 is 120 

120 + 1 = 121

 

Let's have a look at another example.

 

122 + 121 = 

We can double 120  then add 2 + 1 

Double 120 is 240

1 + 2 = 3

240 + 3 = 243

 

Pairs to ten

Where digits add up to 10 we can use this to help.

34 + 46 =

30 + 40 = 70

4 + 6 = 10

70 + 10 = 80

 

Let's have a look at another example.

 

245 + 135 =

240 + 130 = 370

5 + 5 = 10

370 + 10 = 380

 

 

Counting on and counting back

We can use an imaginary number line to count on or back.

23 + 76

Imagine the highest number on a number line (76)

Now, count on 3 - 77, 78, 79  

Next, count on 20  (2 lots of 10) - 89, 99

76 + 23 = 99

 

Let's try counting back.

 

642 - 57

Imagine the highest number on a number line (642)

Count back 7 ( we can count back 2 and 5)

642 - 2 = 640

640 - 5 = 635

Now, we subtract 50

635 - 50 ( we can count back 35 and then 15)

635 - 35 = 600

600 - 15 = 585

 

 

Partitioning the numbers by adding the hundreds, tens, and ones separately, then adding them back together.

235 + 231

200 + 200 = 400

30 + 30 = 60

5 + 1 = 6

400 + 60 + 6 = 466

 

Let's have a look at another example.

 

753 + 262

700 + 200 = 900

50 + 60 = 110

3 + 2 = 5

900 + 110 = 5 = 1,115

 

You might know some other strategies that work well for you! 

Use the strategy that works best for you to answer the following questions - good luck!