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\(\frac{2}{3⋅5}\)+\(\frac{2}{5⋅7}\)+\(\frac{2}{7⋅9}\)+\(\frac{2}{9⋅11}\)+\(\frac{2}{11⋅13}\)

\(\frac{(2*7*9*11*13) + (2*3*9*11*13) + (2*3*5*11*13) + (2*3*5*7*13) + (2*3*5*7*9) }{ (3*5*7*9*11*13)}\)

\(\frac{2*3(7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) }{ 3*5*7*9*11*13}\)

\(\frac{2 (7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) }{ 5*7*9*11*13}\)

\(\frac{2 ( 3003 + 1287 + 715 + 455 + 315 ) }{ 5*7*9*11*13}\)

\(\frac{2 ( 5775) }{ 5*7*9*11*13}\)

\(\frac{2(15)}{9*13}\)

\(\frac{10}{39} \)

Answer is E

Another method is whenever we take LCM of denominator in such questions we know that the highest value in denominator is equal to the multiple of end result denominator.
such as 13 is the highest value in denominator.
the denominator of answer will be multiple of 13 only, and among all options, 39 is the multiple of 13 ( Option E)
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yashikaaggarwal

how is the denominator 13? the denominator is actually 11.13 right and not 13.
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gauri123456
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how is the denominator 13? the denominator is actually 11.13 right and not 13.
I said highest value in denominator is 13 not just 13, its in multiplication with 11.

Plus, I was also confused with that dot thing. Believe me. But its definitely not decimal. In mathematics we sometimes use (dot) in middle to show the multiplication between two numbers. Because in no other way, The value the of the given numbers is 10/39.
You can solve the question using decimal and you will get (0.57+0.35+0.25+0.22+0.18 = 1.57 more than 1) which is not among the option.
Also, you can see the difference between two dots. Decimal ends at bottom this dot is in middle. (•/.) Hopefully its clear.

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Yes thanks a lot. So i assume you are trying to say that since the biggest value denominator is 13, hence the denominator in the answer will also be a multiple of 13 and hence 13. Hope my understanding is correct. Thank you.
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gauri123456
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Yes thanks a lot. So i assume you are trying to say that since the biggest value denominator is 13, hence the denominator in the answer will also be a multiple of 13 and hence 13. Hope my understanding is correct. Thank you.
Yes, its case for most of such questions but you can verify your answer by solving question as well.

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It becomes easy when we factor out common elements.

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Harsh9676
It becomes easy when we factor out common elements.

Posted from my mobile device

Try this way

1/3-1/5 = 2/15 = 2/3.5

That means,

=1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13

=1/3-1/13
=(13-3)/39
= 10/39
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2/3⋅5+2/5⋅7+2/7⋅9+2/9⋅11+2/11⋅13
2*7*9*11*13 + 2*3*9*11*13 + 2*3*5*11*13 + 2*3*5*7*13 + 2*3*5*7*9 / 3*5*7*9*11*13
2*3(7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) / 3*5*7*9*11*13
2 (7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) / 5*7*9*11*13
2 ( 3003 + 1287 + 715 + 455 + 315 ) / 5*7*9*11*13
2 ( 5775) / 5*7*9*11*13
2(15)/9*13
10/39
Answer is E

Another method is whenever we take LCM of denominator in such questions we know that the highest value in denominator is equal to the multiple of end result denominator.
such as 13 is the highest value in denominator.
the denominator of answer will be multiple of 13 only, and among all options, 39 is the multiple of 13 ( Option E)

Hi! yashikaaggarwal

Do we have any other alternate method which can consumer lesser time than this one has consumed?

I tried using the above method, but it is taking quite a longer time, which is not ideal for GMAT.

Regards,
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2/3⋅5+2/5⋅7+2/7⋅9+2/9⋅11+2/11⋅13
2*7*9*11*13 + 2*3*9*11*13 + 2*3*5*11*13 + 2*3*5*7*13 + 2*3*5*7*9 / 3*5*7*9*11*13
2*3(7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) / 3*5*7*9*11*13
2 (7*3*11*13 + 9*11*13 + 5*11*13 + 5*7*13 + 5*7*9) / 5*7*9*11*13
2 ( 3003 + 1287 + 715 + 455 + 315 ) / 5*7*9*11*13
2 ( 5775) / 5*7*9*11*13
2(15)/9*13
10/39
Answer is E

Another method is whenever we take LCM of denominator in such questions we know that the highest value in denominator is equal to the multiple of end result denominator.
such as 13 is the highest value in denominator.
the denominator of answer will be multiple of 13 only, and among all options, 39 is the multiple of 13 ( Option E)

Hi! yashikaaggarwal

Do we have any other alternate method which can consumer lesser time than this one has consumed?

I tried using the above method, but it is taking quite a longer time, which is not ideal for GMAT.

Regards,
the one i stated in alternative solution took me exactly 5 sec to analyze so i guess that's the easiest and quickest solution.
if you are confused regarding before the alternative solution one, yes it quiet take some time, but you can always consider the alternate option which states that denominator of the answer ( in this case 10/39) is a multiple of highest denominator in the question (11*13) so its easy.

Also you can always take common from two number solving that from third or what the other aspirants did is quiet a feasible solution as well (i.e. 2/3*5 = 2/15 or 1/3-1/5 and so on) if we had other options as a multiple of 13 in the options denominator. but since we don't have that case we can choose answer by alternate method easily.
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