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 Post subject: Need help on Y4 Add/Subtract Problem QuestionPosted: Sun Aug 28, 2011 1:40 pm

Joined: Tue Jul 31, 2007 9:37 am
Posts: 408
Hi

DS1 and I are thrown with the following question. Can anyone shed light on the rationale, please:

Library A had 86 more books than library B. 12 books were moved from library A to library B. How many fewer books were there in library B than in library A now?

NB, the key is in the last line - how many fewer books in library B (As Is situation) than library A (To Be situation).

FYI, I think the answer is 62

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 Post subject: Re: Need help on Y4 Add/Subtract Problem QuestionPosted: Sun Aug 28, 2011 4:04 pm

Joined: Wed Aug 25, 2010 2:58 pm
Posts: 496
Yes, I agree with your maths!

So if Library A has 86 more books than Library B the easiest way of showing this is...
A=86
B=0

Then 12 books are taken from A and given to B
A=74
B=12

So the difference is 62!

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 Post subject: Re: Need help on Y4 Add/Subtract Problem QuestionPosted: Thu Sep 01, 2011 8:59 pm

Joined: Tue Jul 21, 2009 9:56 pm
Posts: 8228
It's quite a good question. Maths is right. If it's any help to the child you can think about it slightly differently, but get same answer. If you move 12 books from library A to library B you increase B by 12 and decrease A by 12 so you change the difference between the two libraries by 24.

You are reducing the difference as originally A had 86 more than B. So the new difference is 86-24 = 62. I guess many children will give 74 as the answer as when working quickly one might think that when you shift one book from A to B you change the difference by 1, whereas you change the difference by 2 as one goes up by one and one goes down by 1. Could do it with smaller numbers of biscuits on two plates if child is not convinced. Every time you move one biscuit from one plate to the other the difference between the two plates changes by 2.

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