Åk 6–9

A very useful method to calculated addition is called written mental calculation. You need a paper and pencil in order to write down a simplification in order to do calculations in your head. There are many ways to do this, and no single way is correct. You may find your own method that works for you.

Here below are a few examples of different methods you can use for written mental calculations:

A  Calculating by sorting each number by itself

44 + 39 + 52

Here we add the tens and ones each on their own!

44 + 39 + 52 = (40 + 30 + 50) + (4 + 9 + 2) = 120 + 15 = 135

B  Moving over ones

256 + 595

Here we see from the numbers that five is missing for 595 to be an even 600. We “move” 5 from 256 to 595.

256 + 595 = 251 + 5 + 595 = 251 + 600 = 851 C  Changing the numbers order

11 + 4 + 2 + 26

Here you look at the number and see if some numbers fit together so that you can get tens or whole number which simplify addition.

11 + 4 + 2 + 26 = 30 + 13 = 43 2.75 + 2.4 + 8.25 = 2.4 + (2.75 + 8.25) = 2.4 + 11 = 13.4   