Saturday, 8 September 2012

Introducing MAB Addition and Subtraction



Ever watched, really watched a young mathematician complete an addition problem without intervening? Try, lets say 247 plus 345. Not too difficult right? Ever asked them, without judgement, why are you putting that little one up the top (or bottom)?

Child: You add the 7 and 5 first. 
Me: why do we start at the right with the ones?
Child: You just do, that is 12. So you put a little one under the four and write two down.
Me: why do we put that little one under the 4? Doesn't that make it 14 or 41 now?
Child: No, now it is 4 plus 4 plus 1. 
Me: Okay.
Child: So that is 9 (writes 8 down) and then 2 and 3 are 5. 
Me: So the answer is ...?
Child: Five Nine Two
Me: Five nine two?
Child: Yeah.

MAB is a physical way to demonstrate what is actually happening when we complete these types of addition problems with regrouping... regrouping... that is what is called when you add the 7 and 5 and get 12 and put the "little" 'One' in the next column.

Some students don't know WHY they put the little one there, they just do it because that is where they have been told to put it. They don't recognise that the numbers they are adding can be pulled apart, 200+40+7.... and 300+40+5.... So when we line these up in columns the answer would be 500+80+ 12. 

As for subtraction, why all of that crossing out you might ask. What's all this borrowing? My mother was taught the add ten here, add ten there method... whoa! Wasn't that confusing as a seven year old trying to do MY homework in Mum vs School methods!  

As I always harp on about iPad should never replace a hands on physical method. Have a read of my previous blog post on MAB and using manipulatives to teach place value.

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