Wow! Now this does need some concentration, but is by no means impossible.
Let’s look at an example: 239 + 421
First of all it’s nearly always easier to start with the larger number, so let’s reverse the question.
421 + 239.
Now, start with the 421 and add on 200 to make 621.
Take the 621 and add the tens, that’s 30 to make 651
Finally add 9 to the 651 to make 660.
It can be done!!
Sometimes children may need to make jottings: write down the answer to each stage of the question.
There are times when written methods are applied, it will depend on the circumstances.
Free Y6 maths worksheets: Mentally add 3-digit numbers (pg 1)
Free Y6 maths worksheets: Mentally add 3-digit numbers (pg 2)