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 Post subject: subtraction
PostPosted: Fri Feb 29, 2008 12:32 pm 
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Joined: Thu Jan 03, 2008 8:26 am
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Location: Watford, Herts
My daughter's primary school has standardized on a different method of subtraction from the carries that we'd taught her. For example, given
Code:
  36
- 19

they are told to obtain an extra ten for the 6 by crossing out the 3 and replacing it with a 2. I suppose it gives a better intuition about "borrowing the ten", but it will be problematic when they come to 10001 - 23. Is this method common?


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 Post subject:
PostPosted: Fri Feb 29, 2008 12:37 pm 
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Location: Wirral
Yes it has been in use for a good while now. I used that method when teaching in the 80s. Something to do with the 'borrow and pay back' method not being possible if done practically. With a complicated subtraction it is initially difficult borrowing from several columns away, but once done the rest of the calculation is then easier to complete.


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PostPosted: Fri Feb 29, 2008 12:42 pm 
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I can remember some schools taught this method years ago; so it's not totally new.

Adding to the top and to the bottom is the way I was taught, and is I think much quicker.


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PostPosted: Fri Feb 29, 2008 1:52 pm 
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Joined: Mon Jun 18, 2007 2:32 pm
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Location: East Kent
decomposition

you can't take 9 away from 6.

take a ten from the next column to make 16 in top row.

16 - 9 is possible.

move to tens column. You have already taken 1 ten out so ther are now only 2 tens i.e twenty .


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 Post subject:
PostPosted: Fri Feb 29, 2008 6:03 pm 
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Joined: Mon Feb 12, 2007 1:21 pm
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use a number line - vertical subtraction is not necessary and confuses most children. Sums are never set out vertically in KS2, KS3 or GCSE exams -

http://www.mad4maths.com/parents/


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 Post subject:
PostPosted: Sat Mar 01, 2008 9:30 am 
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Hi yoyo123

The "decomposing" method results in the child being given 18 - 9 as the most difficult possible sum. I am sure that most teachers could teach children that 18 - 9 = 9

I was taught this method from 1963 all the way through schooling and never cam across any other method.

When teaching maths I have always felt that the easiest method is the best.

Regards

Mike

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 Post subject:
PostPosted: Sat Mar 01, 2008 11:24 am 
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Joined: Mon Jun 18, 2007 2:32 pm
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Location: East Kent
I hated decomposition , I was taught an entirely different method (take bottom number away from ten add result to teh top number if it is more than ninne put the "tens" digit in next column).

I had to learn decomposition by rote to teach iot and still got stuck!

I'm OK now but always use my method in my head when going round class marking as it comes naturally to me.


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 Post subject:
PostPosted: Sat Mar 01, 2008 12:04 pm 
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I teach adult learners now, and use several methods depending on how old they are, as the different ways seem to correspond to different generations. I am from the 'borrow and pay back' era, along with the older members in the group!


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 Post subject:
PostPosted: Sat Mar 01, 2008 12:12 pm 
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Please look at using the blank number line - it is so easy - both 'borrowing' and 'decomposition' are likely to be forgotten and applied incorreclty.

It is very hard to go wrong with a blank number line as you are aware of the right size of the answer ....


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 Post subject:
PostPosted: Sat Mar 01, 2008 12:57 pm 
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Joined: Thu Jan 03, 2008 8:26 am
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Location: Watford, Herts
Wallasey wrote:
I am from the 'borrow and pay back' era, along with the older members in the group!

:lol: So for the benefit of this old fossil, in the subtraction 21003 - 14638, is the method to cross out the 100 and write 099?


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