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 Post subject: Help with a fractions question please!Posted: Wed Jul 08, 2009 1:32 pm

Joined: Thu Sep 04, 2008 10:48 am
Posts: 60
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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 Post subject: Posted: Wed Jul 08, 2009 2:05 pm

Joined: Mon Feb 09, 2009 1:12 pm
Posts: 44
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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 Post subject: Posted: Wed Jul 08, 2009 2:18 pm

Joined: Fri Jul 06, 2007 8:31 pm
Posts: 1245
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.

_________________
Animis opibusque parati

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 Post subject: Posted: Wed Jul 08, 2009 2:36 pm

Joined: Thu Sep 04, 2008 10:48 am
Posts: 60
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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 Post subject: Posted: Wed Jul 08, 2009 4:11 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 13017
Which tests is he sitting?

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 Post subject: Posted: Wed Jul 08, 2009 4:23 pm

Joined: Thu Sep 04, 2008 10:48 am
Posts: 60
Location: Finchley
He's sitting the exams for independent boys schools in London next January.

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