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Which of the following could be the sum of the reciprocals

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Which of the following could be the sum of the reciprocals [#permalink] New post 31 Mar 2009, 20:29
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Which of the following could be the sum of the reciprocals of two different prime numbers?

A. 7/13
B. 10/12
C. 11/30
D. 23/50
E. 19/77
[Reveal] Spoiler: OA
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Re: sum of reciprocals. [#permalink] New post 31 Mar 2009, 21:07
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Re: sum of reciprocals. [#permalink] New post 02 Jan 2011, 21:23
nitya34 wrote:
10/12 it is
10/12=5/6=(1/2) + (1/3) :)


Was it a guess ??? or how do we approach to this kind of problem...
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Re: sum of reciprocals. [#permalink] New post 03 Jan 2011, 00:41
are there any theories for these kinds of problems? haven't came across any yet, i think hit and trial is the best one!
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Re: sum of reciprocals. [#permalink] New post 03 Jan 2011, 01:53
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jullysabat wrote:
nitya34 wrote:
10/12 it is
10/12=5/6=(1/2) + (1/3) :)


Was it a guess ??? or how do we approach to this kind of problem...

prab wrote:
are there any theories for these kinds of problems? haven't came across any yet, i think hit and trial is the best one!


Which of the following could be the sum of the reciprocals of two different prime numbers?
A. 7/13
B. 10/12
C. 11/30
D. 23/50
E. 19/77

Let x and y be two different primes, then their sum will be \frac{1}{x}+\frac{1}{y}=\frac{x+y}{xy}. So, the denominator of the reduced fraction must be the product of two different primes: only B, \frac{10}{12}=\frac{5}{6}=\frac{5}{2*3}, and E, \frac{19}{77}=\frac{19}{7*11}, have such denominators and nominator must be the sum of the primes in denominator, thus only B is left: \frac{10}{12}=\frac{5}{6}=\frac{2+3}{2*3}.

Answer: B.

Hope it's clear.

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Re: sum of reciprocals. [#permalink] New post 03 Jan 2011, 10:00
Bunuel wrote:
jullysabat wrote:
nitya34 wrote:
10/12 it is
10/12=5/6=(1/2) + (1/3) :)


Was it a guess ??? or how do we approach to this kind of problem...

prab wrote:
are there any theories for these kinds of problems? haven't came across any yet, i think hit and trial is the best one!


Which of the following could be the sum of the reciprocals of two different prime numbers?
A. 7/13
B. 10/12
C. 11/30
D. 23/50
E. 19/77

Let x and y be two different primes, then their sum will be \frac{1}{x}+\frac{1}{y}=\frac{x+y}{xy}. So, the denominator of the reduced fraction must be the product of two different primes: only B, \frac{10}{12}=\frac{5}{6}=\frac{5}{2*3}, and E, \frac{19}{77}=\frac{19}{7*11}, have such denominators and nominator must be the sum of the primes in denominator, thus only B is left: \frac{10}{12}=\frac{5}{6}=\frac{2+3}{2*3}.

Answer: B.

Hope it's clear.


Hey Bunnel,

Thank u so much.. for this explanation...
very very simple and robust way of explaning things...
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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 11 Jun 2013, 08:22
Hi Bunuel

Why not the ans would be 11/30? Pls explain

If we take 2 prime number as 5 and 6 then by equation it is 5+6/5*6=11/30 ans.....

Apart from this PS question I have a query..that is currently I am doing practice from GMAT club. Is it enough or I must follow some other source ?

Needless to say +Kudos to your simplistic approach for every question.

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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 11 Jun 2013, 08:28
[quote="prasannajeet"]Hi Bunuel

Why not the ans would be 11/30? Pls explain

If we take 2 prime number as 5 and 6 then by equation it is 5+6/5*6=11/30 ans.....



Because 6 is not a prime)
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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 11 Jun 2013, 08:29
Expert's post
prasannajeet wrote:
Hi Bunuel

Why not the ans would be 11/30? Pls explain

If we take 2 prime number as 5 and 6 then by equation it is 5+6/5*6=11/30 ans.....

Apart from this PS question I have a query..that is currently I am doing practice from GMAT club. Is it enough or I must follow some other source ?

Needless to say +Kudos to your simplistic approach for every question.

Rgds
Prasannajeet


6=2*3 is not a prime number.

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PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 11 Jun 2013, 08:38
Expert's post
prasannajeet wrote:

Apart from this PS question I have a query..that is currently I am doing practice from GMAT club. Is it enough or I must follow some other source ?


I think that we have more than enough questions and theory topics on the site. Though you can check Best GMAT Books:
best-gmat-math-prep-books-reviews-recommendations-77291.html
books-to-read-improve-verbal-score-and-enjoy-a-good-read-76079.html

Hope it helps.

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PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 11 Jun 2013, 09:53
Hi Sivtozar & Bunuel

Thank you . I made a felony not a mistake to identify the prime number.
Also thanx for the thread.

Rgds
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Re: Which of the following could be the sum of the reciprocals [#permalink] New post 09 Jul 2014, 08:19
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Re: Which of the following could be the sum of the reciprocals   [#permalink] 09 Jul 2014, 08:19
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