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PostPosted: Wed Jul 08, 2009 1:32 pm 
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Joined: Thu Sep 04, 2008 10:48 am
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Location: Finchley
Can someone help me explain how to do this question which was on a practice test my son was given?

If 1/a + 1/b = 7/10
What are the values of "a" and "b"?

Personally I haven't a clue, so any help much appreciated!


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PostPosted: Wed Jul 08, 2009 2:05 pm 
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1/a + 1/b = 7/10


1*b + 1*a ...........7
_________ = _____
a*b ....................... 10


a+b .................. ..................... 7
_____ ...................= ................. ______
a * b ........................................ 10

These being equivalent fractions


a+b = 7 and a*b = 10

If you now find factors of 10 such that they add upto 7, which is the answer

a, b will be 5,2


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PostPosted: Wed Jul 08, 2009 2:18 pm 
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or by observing split into two terms...

7/10

is...

6/10 + 1/10 which is... 3/5 + 1/10... i.e. not of the form 1/a+1/b so ignore...

5/10 + 2/10 which is... 1/2 + 1/5... Looks Good

4/10 + 3/10 which is... 2/5 + 3/10... i.e. not of the form 1/a+1/b so ignore...

3/10 + 4/10 ... hang on... we've done this one so STOP.

_________________
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PostPosted: Wed Jul 08, 2009 2:36 pm 
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Location: Finchley
Oh boy, don't think I will be able to explain it very easily with confusing my son! I guess the split terms method would be the easiest way to go, as he should at least know that the denominator has to be the same when adding fractions.

Do these types of questions come up very often in 11+ exams or can I just ignore it and hope for the best? (she asks, with fingers crossed)


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PostPosted: Wed Jul 08, 2009 4:11 pm 
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Which tests is he sitting?


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PostPosted: Wed Jul 08, 2009 4:23 pm 
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Location: Finchley
He's sitting the exams for independent boys schools in London next January.


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