Maths Update: Adding/Subtracting 2-digit numbers

After revising partitioning, we recapped how to add 2-digit numbers mentally, i.e. partition into Tens and Units, add the Tens, add the Units, add both parts together. They also had the option of adding the Tens from one number to the other, then counting up the remaining Units, e.g.

28 + 43 =

20 + 40 = 60
8 + 3 = 11

60 + 11 =

60 + 10 = 70
0 + 1 = 1

70 + 1 = 71 so 28 + 43 = 71

OR

28 + 40 = 68
68 + 3 = 71 so 28 + 43 = 71

We concentrated on the first method first, to get them used to partitioning the numbers. Then we moved onto the second method as something more efficient.

After the mental practise, I introduced the column method to them as an efficient way of writing their working out down. It's the same principle, partitioning and adding each place value one at a time, but written in a compact way, e.g.

16 + 43 = 

   16
+ 43
   59

We started with numbers which didn't carry over into the next column, then once they were comfortable with the method I explained how to write the extra Tens underneath the Tens column - and to make sure they included them in their answer! e.g.

45 + 36 =

    45
 + 36
    81
    1

We also spoke about using a quick mental method to check their answers.

We did all the above through explanation and examples on the whiteboard first, then answering questions from a photocopied worksheet using the method we were focusing on at the time.

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Next, we recapped subtraction of 2-digit numbers via partitioning and counting down, e.g.

38 - 14 =

38 - 10 = 28
28 - 4 = 24 so 38 - 14 = 24

Where the numbers stayed within the Tens, then using numbers in the Unit column which meant counting down across Tens, e.g.

52 - 26 = 

52 - 20 = 32
32 - 6 = 26 so 52 - 26 = 26

After some practise doing this mentally, I then introduced the column method to them - but for numbers which wouldn't require carrying, e.g.

88 - 43 =

    88
  - 43
    45

I decided that was enough for now, so they could get used to the method, and we would look at carrying from the Tens column at a later point inshaAllah!

The above was all done, one method at a time on separate days, first on the whiteboard and then in the context of numbers by working through practise pages from one of the workbooks we have.

Finally, I gave them some word problems to work through so they could use these skills in the context of money. They had the choice to use either a mental or a written method, whichever they found easier.