We have spent the last two weeks working in Subtraction Groups. Each group has been focusing on building speed with subtraction facts under 20 and different strategies for solving subtraction equations. In Year Two it is expected that students can use a number of different strategies to solve equations.
We have explored using concrete materials, such as number charts and bundles of craft sticks. We have used number lines, vertical recording and decomposing. These were the same strategies we used when working with addition. To ensure that we don’t confuse the two operations we have also been exploring number stories to help us visualise what we are doing.
In subtraction there are three types of stories we can explore:
- take away (what we are most familiar with)
- missing part
The reason we have used craft sticks is to help us understand the need for re-grouping (or what parents might remember as trading).
Take the number 32. If the equation was 32 – 18, we know that 32 is a bigger number but there are only 2 single sticks. When we look at the ones and see 2-8 we can get a bit confused. Many students want to reverse the numbers in the ones, calculating 8 – 2 instead of 2 -8. This is where the bundles help…
In order for us to be able to subtract 8 we need to unbundle, changing 32 (3 tens and 2 ones) into 20 & 12 (2 tens and 12 ones). Now we can take 8 ones away and one ten to get the final answer.
Once we understand the need to unbundle or re-group, we then better understand the vertical recording of such equations.
I wonder if you can show mum and dad some of the strategies you have been practicing? Which is your favourite and why?