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 Post subject: Help maths question
PostPosted: Tue Oct 14, 2008 7:11 pm 
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Joined: Tue Oct 14, 2008 7:06 pm
Posts: 3
Can anyone help with a quick method to work out questions like the one below:

Two Whole number are each between 50 and 70
They multiple to make 4095
write in the missing numbers
___ X ___ = 4095


thanks


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 Post subject:
PostPosted: Tue Oct 14, 2008 7:55 pm 
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Joined: Thu Jul 10, 2008 9:56 pm
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Think of the units....... The "unit" digit of one number multiplied by the second "unit" digit must result in another number which has 5 for units. As far as I can see one of the numbers has to end in a 5. Therefore you only have to think about two numbers.... 55 and 65 .....

By estimating you can see that it is more likely to be 65 than 55 so try that one first.....

4095 / 65 = 63

The answer will therefore be 63 x 65 = 4095

The "unit" trick is good for checking long multiplication (what I mean by this is if you have a number 4562 x 5467 your answer is bound to end in a 4 because 2 x 7 is 14 ) and also for multiple choice exams although some papers I noticed have all options ending in the same digit!

So, in answer to your question, look for the units first to see if it would give you a clue.


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 Post subject:
PostPosted: Tue Oct 14, 2008 8:10 pm 
I agree that the giveaway was the fact it ended in 5. However, if it did not, trial and error would be harder.

The other way of doing it is to work it out as a product of prime factors.

4095 = 3 x 3 x 5 x 7 x 13

then group accordingly 5 x 13 (65) and 3 x 3 x 7 ( 63).

This technique would work for all questions of this type.


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PostPosted: Wed Oct 15, 2008 8:59 am 
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Joined: Tue Oct 14, 2008 7:06 pm
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Thankyou all for your help. :D


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