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In this activity, we will be adding and subtracting numbers mentally.
Let's remind ourselves what it means to do something mentally.
It means that we work it out in our heads (but don't forget we can always use our fingers to help us!)
We will be adding and subtracting two-digit numbers with two-digit numbers.
Let's have a look at what all this means!
Adding two two-digit numbers
A two-digit number is any number made up of two digits, so that's all the numbers between 10 and 99!
Let's have a look at an example question and how we could answer it:
46 + 23 =
Here are the steps we need to take to answer this mentally:
- Put the biggest number in your head, so that's 46
- Split the 23 into tens and ones: 20 + 3
- First, we are going to add on the tens, there are 2 tens in 20 so hold up 2 fingers.
- Now count along 2 tens from 46: 56, 66
We have landed on 66.
The next thing we need to do is add on the ones.
- There are 3 ones in 23 so we need to hold up 3 fingers and add on 3 to our 66: 67. 68. 69
Our final answer is 69.
Subtracting two two-digit numbers
A two-digit number is any number made up of two digits, so that's all the numbers between 10 and 99!
Let's have a look at an example question and how we could answer it:
42 - 34 =
Here are the steps we need to take to answer this mentally:
- Put the biggest number in your head, so that's 42
- Split the 34 into tens and ones: 30 + 4
- First, we are going to take away the tens, there are 3 tens in 30 so hold up 3 fingers.
- Now count back 3 tens from 42: 32, 22, 12
We have landed on 12.
The next thing we need to do is take away the ones.
- There are 4 ones in 34 so we need to hold up 4 fingers and take away 4 from our 12: 11, 10, 9, 8
Our final answer is 8.
Now, get your brains ready, and let's have a go at some questions!
Remember to read the question very carefully to see whether it is an adding or subtracting question?
