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 Post subject: multiplying fractions?
PostPosted: Wed Aug 19, 2009 2:51 pm 
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Joined: Thu Aug 06, 2009 3:32 pm
Posts: 214
hello could anyone please help me with this fraction problem?
1 2/3 * 3 5/7=
i have completely forgotten what the method is and my ds has not covered it at school yet. i think (and according to dh) you multiply denominators and numerators with each other?
also does anyone know whether this type of problem comes up in 11
plus in surrey.
thanks


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 Post subject:
PostPosted: Wed Aug 19, 2009 7:24 pm 
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Joined: Sat Nov 24, 2007 11:09 pm
Posts: 218
1 2/3 times 3 5/7

Change the mixed fractions into improper fractions first i.e. 1 2/3 = 5/3, 3 5/7 = 26/7

1 2/3 * 3 5/7

= 5/3 * 26/7

= 130/21 ( 6 4/21)


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PostPosted: Wed Aug 19, 2009 7:32 pm 
shafstsbury1 wrote:
hello could anyone please help me with this fraction problem?
1 2/3 * 3 5/7=
i have completely forgotten what the method is and my ds has not covered it at school yet. i think (and according to dh) you multiply denominators and numerators with each other?
also does anyone know whether this type of problem comes up in 11
plus in surrey.
thanks


Convert your sum into 5/3 * 26/7 =
then you can multiply as your DH says -
5*26 = 130
3*7 = 21

so 130/21 = 6 4/21

I dont know if it comes up in Surrey but havent seen that type of q in any papers we have tried. More likely to be something like 1/3 * 1/4 as that would have been covered in school I think

rosered


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 Post subject:
PostPosted: Wed Aug 19, 2009 7:33 pm 
Oooh :oops: - took too long to write the post!


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 Post subject:
PostPosted: Wed Aug 19, 2009 8:44 pm 
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Joined: Thu Jan 03, 2008 8:26 am
Posts: 1326
Location: Watford, Herts
Although it doesn't come up in this example, fractions must always be reduced to the simplest form (by dividing by common factors) for full marks. It's a good idea to divide before multiplying, as then you're multiplying smaller numbers (and not dividing big numbers). e.g.
Code:
                 21   15   21 x 15   3 x 3   9
2 1/10 x 2 1/7 = -- x -- = ------- = ----- = - = 4 1/2
                 10   7    10 x 7    2 x 1   2

edit: removed mention of cancelling


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 Post subject:
PostPosted: Fri Aug 21, 2009 12:40 pm 
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Joined: Thu Aug 06, 2009 3:32 pm
Posts: 214
thank you everyone for your help. i am sorry i am a bit late in responding!
And thank you for the tip on simplifying.
i did think this was an unusual q for 11+ as it is multiplying mixed fractions.


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