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What's the fastest way of doing this Q?

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What's the fastest way of doing this Q?

by capnx » Sat Sep 05, 2009 5:06 pm
Which of the following could be the sum of the reciprocals of two different prime numbers?

a 7/13
b 10/21 (edited)
c 11/30
d 23/50
e 19/77

I can't get my brain around this question. Please help. Thanks
Last edited by capnx on Sat Sep 05, 2009 6:11 pm, edited 1 time in total.
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by bharathh » Sat Sep 05, 2009 5:16 pm
Are the answer choices correct?

Regardless here's my solution for this.

Eliminate the choices where the denominator is not a product of 2 prime numbers because when you add the reciprocal of 2 prime numbers e.g 2 and 3

1/2 + 1/3 , the denominator will be 2*3 = 6

Only 77 = 7*11 and 21 = 3*7 are possibly valid answers.

1/7 + 1/11 = 11+7/77 = 18/77
1/3 + 1/7 = 3+7/21 = 10/21

Neither choice is in the answer list
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by capnx » Sat Sep 05, 2009 6:11 pm
i'm sorry, B should be 10/21
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by bharathh » Sat Sep 05, 2009 6:23 pm
:D

Answer is B then. Did my explanation help you?
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by capnx » Sat Sep 05, 2009 7:39 pm
yes, very helpful. Thanks :)
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