subtraction

11 Plus Maths – Preparation and Information

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yoyo123
Posts: 8099
Joined: Mon Jun 18, 2007 3:32 pm
Location: East Kent

Post by yoyo123 »

I'm a fan of number lines too Guest55!

especially with children who have difficulty with maths. Rulers make really good impromptu number lines (metre rules too!)

The numeracy strategy used number lines a lot when it was introduced.

I had a washing line in my classroom and we pegged numbers on it..
I'm a believer in kinaesthetic stuff
Mike
Posts: 625
Joined: Thu Jan 12, 2006 4:29 pm

Post by Mike »

Hi WP

21003 - 14638

in columns

2 1 0 0 3
1 4 6 3 8

There are two ways of verbalising the question

1. Is to say three take away eight, can't do
or
2. Eight from three, can´t do.

I prefer the first, because it is worded in the correct order of the question.

You need to make 3 bigger than 8, by converting numbers into their next lowest value.

i.e. A ten becomes ten units, one hundred becomes ten tens, one thousand becomes 10 hundreds.

So, the 1 in the thousand column becomes 0, making the 0 in the hundreds column 10.

The 10 in the hundreds column becomes 9, making the 0 in the tens column 10.

The 10 in the tens column becomes 9 and the 3 in the units column becomes 9.

Giving the following

2 0 9 9 13
1 4 6 3 8

You cab now work out, from right to left, the first part of the sum

13 - 8 = 5
9 - 3 = 6
9 - 6 = 3

Next, the 2 in the 10,000 column becomes 1 and the 0 in the thousand column becomes 10, giving the next part of the sum

10 - 4 = 6
and
1 - 1 = 0

Giving the answer 6365

Regards

Mike
Mike Edwards is a co-author of The Tutors product range.
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Post by Guest55 »

This method totally ignores place value of the digits and it is not the 'best' method according to experts in Maths education.

Numbers like this would never come up in KS2 and would be on a calculator paper at a later stage -
Mike
Posts: 625
Joined: Thu Jan 12, 2006 4:29 pm

Post by Mike »

Which experts are these?

The CIMT site is not accessible at the moment.

Rgards

Mike
Mike Edwards is a co-author of The Tutors product range.
WP
Posts: 1331
Joined: Thu Jan 03, 2008 9:26 am
Location: Watford, Herts

Post by WP »

Guest55 wrote:Numbers like this would never come up in KS2 and would be on a calculator paper at a later stage -
But we still want our children to be able to do such sums, don't we?
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Post by Guest55 »

Not on paper - pupils are taught to use appropriate methods. For that question a calculator is the appropriate method -
WP
Posts: 1331
Joined: Thu Jan 03, 2008 9:26 am
Location: Watford, Herts

Post by WP »

Guest55 wrote:Not on paper - pupils are taught to use appropriate methods. For that question a calculator is the appropriate method -
Gosh, I must be old!
aliportico
Posts: 888
Joined: Wed Aug 01, 2007 12:19 pm

Post by aliportico »

Lol, me too then!

Not everything's about school and tests, is it? Or are we supposed to spend the rest of our lives with a calculator in our pockets, ready to use it in appropriate situations? I'd like my children to know how to do sums like that in their heads and on paper, and so far 3/4 of them do. (Just the 5 year old to go!)
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Post by Guest55 »

Give me a real life example when you would need to do that sum?
aliportico
Posts: 888
Joined: Wed Aug 01, 2007 12:19 pm

Post by aliportico »

Doing £210.03 - £146.38 for some reason? I've certainly done similar calculations to that recently - transferring money to and from bank accounts, for example - I can see a couple on my desk in front of me, lol.

Or reading your electricity meter, and wanting to work out how many units you've used.

Or keeping track of the mileage on your car so you know how far you've gone on a tank of petrol.

Etc.
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