In an electric circuit, two resistors with resistances x and

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In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the reciprocals of the sum of x and y. What is r in terms of x and y ?

(A) xy
(B) x + y
(C)1/(x + y)
(D) xy/(x + y)
(E) (x + y)/xy
Last edited by AsadAbu on Sun Oct 18, 2015 9:15 am, edited 2 times in total.
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by visver » Thu Oct 15, 2015 11:06 pm
1/r = 1/x + 1/y
or, 1/r = (x+y)/xy
or, r = xy/(x+y)

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by AsadAbu » Thu Oct 15, 2015 11:11 pm
Hi Visver, Your calculation is NOT correct!!

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by [email protected] » Thu Oct 15, 2015 11:41 pm
Hi iMyself,

Visver's solution IS correct. This question can be found in the OG, so if you think that the provided solution is incorrect, then where did you find it?

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by AsadAbu » Fri Oct 16, 2015 12:02 am
[email protected] wrote:Hi iMyself,

Visver's solution IS correct. This question can be found in the OG, so if you think that the provided solution is incorrect, then where did you find it?

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You are ALSO wrong. So, please think about it for the second time!

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by [email protected] » Fri Oct 16, 2015 12:17 am
HI iMyself,

This question appears in the OG13/GMAT2015/GMAT2016 on page 183 (although with slightly different question numbers). You can also feel free to read the explanation offered in any of those books. They all confirm the same answer.

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by AsadAbu » Fri Oct 16, 2015 2:31 am
[email protected] wrote:HI iMyself,

This question appears in the OG13/GMAT2015/GMAT2016 on page 183 (although with slightly different question numbers). You can also feel free to read the explanation offered in any of those books. They all confirm the same answer.

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Rich
So what? This is NOT your job to make comparison among those official guide edition (where is located this question); your job is to solve the question by seeing the question pattern!

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by Brent@GMATPrepNow » Fri Oct 16, 2015 6:56 am
iMyself wrote: So what? This is NOT your job to make comparison among those official guide edition (where is located this question); your job is to solve the question by seeing the question pattern!
Now that you've piqued our interest, what is the "correct" answer, and how is it derived?
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by AsadAbu » Fri Oct 16, 2015 9:19 pm
Brent@GMATPrepNow wrote:
iMyself wrote: So what? This is NOT your job to make comparison among those official guide edition (where is located this question); your job is to solve the question by seeing the question pattern!
Now that you've piqued our interest, what is the "correct" answer, and how is it derived?
It's a simple math. I am astonished-why do you people can not solve it. Please read the question carefully.

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by [email protected] » Sat Oct 17, 2015 9:30 am
Hi All,

The original prompt actually differs from the one listed in the OGs (so it was either transcribed incorrectly or purposely changed).

In the OGs, the prompt states that "the reciprocal of R is equal to the SUM of the RECIPROCALS of X and Y."

In the post here, the prompt states that "the reciprocal of R is equal to the RECIPROCAL of the SUM of X and Y."

EVERYTHING else (including the 5 answer choices) is exactly the same though, so I'm led to believe that this question was mis-transcribed. With this transcription error, the "correct" answer would change though - it would become B.

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by GMATGuruNY » Sun Oct 18, 2015 3:01 am
iMyself wrote:In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the reciprocal of the sum of x and y. What is r in terms of x and y ?

(A) xy
(B) x + y
(C)1/(x + y)
(D) xy/(x + y)
(E) (x + y)/xy
I received a PM requesting my input.

The reciprocal of r is equal to the reciprocal of the sum of x and y.
1/r = 1/(x+y).

Cross-multiplying, we get:
x+y = r.

The correct answer is B.

The problem posted above is an altered version of an official problem in the OG16.
The original problem -- which is trickier -- can be found here:
https://www.beatthegmat.com/reciprocals-t84790.html
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by AsadAbu » Mon Oct 19, 2015 9:56 am
GMATGuruNY wrote:
iMyself wrote:In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the reciprocal of the sum of x and y. What is r in terms of x and y ?

(A) xy
(B) x + y
(C)1/(x + y)
(D) xy/(x + y)
(E) (x + y)/xy
I received a PM requesting my input.

The reciprocal of r is equal to the reciprocal of the sum of x and y.
1/r = 1/(x+y).

Cross-multiplying, we get:
x+y = r.

The correct answer is B.

The problem posted above is an altered version of an official problem in the OG16.
The original problem -- which is trickier -- can be found here:
https://www.beatthegmat.com/reciprocals-t84790.html
There is still something missing in your concept.

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by AsadAbu » Mon Oct 19, 2015 9:58 am
[email protected] wrote:Hi All,

The original prompt actually differs from the one listed in the OGs (so it was either transcribed incorrectly or purposely changed).

In the OGs, the prompt states that "the reciprocal of R is equal to the SUM of the RECIPROCALS of X and Y."

In the post here, the prompt states that "the reciprocal of R is equal to the RECIPROCAL of the SUM of X and Y."

EVERYTHING else (including the 5 answer choices) is exactly the same though, so I'm led to believe that this question was mis-transcribed. With this transcription error, the "correct" answer would change though - it would become B.

GMAT assassins aren't born, they're made,
Rich
I've changed the question pattern intentionally. But, there is still something missing in your concept,too.

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by GMATGuruNY » Mon Oct 19, 2015 10:53 am
iMyself wrote:I've changed the question pattern intentionally..
Please do not rewrite official problems.
Posting altered versions of official problems will serve only to confuse readers of BTG.
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I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
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As a tutor, I don't simply teach you how I would approach problems.
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