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PostPosted: Tue Jun 26, 2012 9:45 am 
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My dd (in year 3) has been doing some Bond 10 minute maths to keep her sister company.

We have had a couple of questions like 227-63=.

She hasn't yet been taught how to do sums formally so can't do it that way (i.e. in columns of hundreds, tens units with carrying etc.).

I went to a talk at school once about how maths is now taught, and so I think at her stage they have to count back from 227 to 63 i.e get to 220 (-7), then 200 (-20), then 100 (-100), then 70 (-30), then 63 (-7); then add up the bits in brackets 7+20+100+30+7= 157.

But I don't have the faintest clue how to present this on the paper. Can anyone enlighten me/ suggest any books that cover it? Or shall I just teach her the formal way and hope her head doesn't drop off..?


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PostPosted: Tue Jun 26, 2012 9:54 am 
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Location: East Kent
I'd use a number line


http://www.rosley.cumbria.sch.uk/homework%203&4.pdf


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PostPosted: Tue Jun 26, 2012 12:11 pm 
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Yeah, number line. I think it's easier to count whilst jumping forwards from 63, but that might just be me. It's useful to be able to think about 227-63 as being the difference between 227 and 63 and know that it doesn't matter which way you go, anyway.


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PostPosted: Tue Jun 26, 2012 3:13 pm 
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Thanks! I did try a number line but she looked at me like I was from another planet, but will persevere if that's what they would be doing at school. (She's supposed to be a level 4 for maths, so I suspect some of it is her having me on, to be honest.. I was probably writing in the wrong colour pen or something..)


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PostPosted: Tue Jun 26, 2012 4:28 pm 
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Why does she have to do this at home? Couldn't you just wait till the school does it, or is she very keen to 'keep her sister company' while she does extra Maths?
Just a thought. :)


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PostPosted: Tue Jun 26, 2012 10:40 pm 
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I think you are confusing 2 methods. Use a number line to count up from 63 to 227 (63 to 100 = 37 and 100 to 227 = 127, 37 + 127 = 164) or count 63 back from 227. Crossing the hundred boundary is usually tricky for average Y3s, so they often count back in 10s (217, 207, 197 etc, then take off the 3).
But to be honest a level 4 mathematician, especially one in Y3, (level 4 in Y3 is a very able mathematician), should be able to do this calculation easily in their head, or use formal methods for more complicated calculations, so you're probably right, she might be having you on! :D


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PostPosted: Thu Jun 28, 2012 8:51 am 
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Hi Amber, she doesn't have to do it, she wants to. She is one of those children that wants to know everything, reads insatiably, consumes puzzle books etc.


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PostPosted: Thu Jun 28, 2012 10:40 am 
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I'm not wishing to sound negative, but if she has no method currently for doing that subtraction she is either having you on big-time, or she is no way level 4. 4b is the expected level at the end of KS2 and they would be able to subtract by more than one method!!

They would teach a child lots of different ways to do subtraction at school these days, and then expect the child to choose the most appropriate method for the calculation they had been given. She should be able to tell you at least one way that she does it. Maybe try her with a something smaller e.g. 85 - 7 and get her to tell you how she does it. Then move on to two digits take away two digits. Then move on to 3 digits minus 1 digit and get her to explain, then 3 digits minus 2 digits and get her to explain.

One way she might have been taught is like this:

227 - 63

First of all do 227 - 60 --------- she could count backwards in tens if needs be - 217 207 197 187 177 167

Then get her to take away the 3 to get 164

The idea is to do it mentally is that first of all you take away the hundreds, then the tens, then the units etc etc - same logic for bigger numbers.

Try ones also where when she gets down to the units she is having to cross the tens boundary so try

227 - 69. She should be able to do it her head by the method above.

Drawing a number line in year 3 is useful when you don't understand it, but a bit of a long-winded phaff if you do.

She doesn't need to be able to do it by the column method as long as she has a method that works fast and accurately for her- and that's true for the 11plus too.


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