Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 25 May 2017, 22:53

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# In an electric circuit, two resistors with resistances x and

Author Message
TAGS:

### Hide Tags

Manager
Joined: 02 Dec 2012
Posts: 178
Followers: 6

Kudos [?]: 2695 [0], given: 0

In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

29 Dec 2012, 05:39
11
This post was
BOOKMARKED
00:00

Difficulty:

15% (low)

Question Stats:

68% (01:52) correct 32% (01:01) wrong based on 904 sessions

### HideShow timer Statistics

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 sum of the reciprocals 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
[Reveal] Spoiler: OA
Math Expert
Joined: 02 Sep 2009
Posts: 38890
Followers: 7735

Kudos [?]: 106171 [0], given: 11607

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

29 Dec 2012, 05:42
Expert's post
1
This post was
BOOKMARKED
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 sum of the reciprocals 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

The wording is a bit confusing, though basically we are told that 1/r = 1/x + 1/y, from which it follows that r=xy/(x + y).

_________________
Manager
Joined: 03 Nov 2009
Posts: 65
Followers: 1

Kudos [?]: 14 [1] , given: 17

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

29 Dec 2012, 05:54
1
KUDOS
From the statements:
x+y = r --> 1
1/r = 1/x+1/y --> 2

From 1 and 2

So 1/r = (x+y)/xy,

r = xy/(x+y)

Ans - D
Manager
Joined: 03 Nov 2009
Posts: 65
Followers: 1

Kudos [?]: 14 [0], given: 17

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

29 Dec 2012, 05:54
From the statements:
x+y = r --> 1
1/r = 1/x+1/y --> 2

From 1 and 2

So 1/r = (x+y)/xy,

r = xy/(x+y)

Ans - D
Manager
Joined: 12 Jan 2013
Posts: 220
Followers: 5

Kudos [?]: 72 [0], given: 47

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

27 Dec 2013, 10:31
Bunuel 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 sum of the reciprocals 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

The wording is a bit confusing, though basically we are told that 1/r = 1/x + 1/y, from which it follows that r=xy/(x + y).

Could you explain this more in depth?

I interpret it as:

" r is the combined of x and y" --> r = x + y

"the reciprocal of r is equal to the sum of the reciprocals of x and y" ---> 1/r = (1/x) + (1/y)

" What is r in terms of x and y?" ---> 1 = [(1/x) + (1/y)] * r ----> r = 1 * [(x/1) + (y/1)] ---> r = x + y

Where's the flaw in my calculation?
Math Expert
Joined: 02 Sep 2009
Posts: 38890
Followers: 7735

Kudos [?]: 106171 [1] , given: 11607

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

28 Dec 2013, 03:42
1
KUDOS
Expert's post
aeglorre wrote:
Bunuel 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 sum of the reciprocals 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

The wording is a bit confusing, though basically we are told that 1/r = 1/x + 1/y, from which it follows that r=xy/(x + y).

Could you explain this more in depth?

I interpret it as:

" r is the combined of x and y" --> r = x + y

"the reciprocal of r is equal to the sum of the reciprocals of x and y" ---> 1/r = (1/x) + (1/y)

" What is r in terms of x and y?" ---> 1 = [(1/x) + (1/y)] * r ----> r = 1 * [(x/1) + (y/1)] ---> r = x + y

Where's the flaw in my calculation?

$$\frac{1}{r} = \frac{1}{x} + \frac{1}{y}$$;

$$\frac{1}{r} = \frac{y+x}{xy}$$;

$$r=\frac{xy}{x+y}$$.

Hope it's clear.
_________________
Manager
Joined: 12 Jan 2013
Posts: 220
Followers: 5

Kudos [?]: 72 [0], given: 47

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

28 Dec 2013, 05:15
Bunuel wrote:

$$\frac{1}{r} = \frac{1}{x} + \frac{1}{y}$$;

$$\frac{1}{r} = \frac{y+x}{xy}$$;

$$r=\frac{xy}{x+y}$$.

Hope it's clear.

Bunuel, this is still not clear to me.

I remember asking this very same question (in another thread) about a week ago. Quite obviously there are flaws in my fundamental understanding of concepts in regards to addition of algebraic fractions. Can you direct me to some guide or tutorial that in a simple but efficient way explains these concepts?

Or rather, if you - as you read this - instantly understand where my flaws are and what rule/law/fundamental error I need to "fix", I would be very thankfull if you could explain to me what understanding I lack for algebraic addition of fractions.

By the way, thank you for all of your help!
Intern
Joined: 17 Feb 2014
Posts: 1
Followers: 0

Kudos [?]: 6 [1] , given: 48

In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

22 Oct 2014, 07:56
1
KUDOS
aeglorre wrote:
Bunuel 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 sum of the reciprocals 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

The wording is a bit confusing, though basically we are told that 1/r = 1/x + 1/y, from which it follows that r=xy/(x + y).

Could you explain this more in depth?

I interpret it as:

" r is the combined of x and y" --> r = x + y

"the reciprocal of r is equal to the sum of the reciprocals of x and y" ---> 1/r = (1/x) + (1/y)

" What is r in terms of x and y?" ---> 1 = [(1/x) + (1/y)] * r ----> r = 1 * [(x/1) + (y/1)] ---> r = x + y

Where's the flaw in my calculation?

The steps should be as follows:

-> 1 = [(1/x)+(1/y)] * r
-> r = 1 / [(1/x)+(1/y)]
-> r = 1/ [(y+x)/xy] ... addition in denominator
-> r = 1/ [(x+y)/xy]
-> r = 1/1 * [xy/(x+y)] ... dividing fraction using reciprocal (flipping)
-> r= [xy/(x+y)]
Director
Joined: 10 Mar 2013
Posts: 597
Location: Germany
Concentration: Finance, Entrepreneurship
GMAT 1: 580 Q46 V24
GPA: 3.88
WE: Information Technology (Consulting)
Followers: 17

Kudos [?]: 356 [0], given: 200

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

20 May 2015, 13:10
after some manipulations you get 1/R = x+y/xy you can just reverse this equation to get R --> R = xy/x+y
And yes, if a question is whether we can just reverse the equation -- 1/2 = 2/4 --> 2/1 = 4/2 (Both parts =2)
_________________

When you’re up, your friends know who you are. When you’re down, you know who your friends are.

800Score ONLY QUANT CAT1 51, CAT2 50, CAT3 50
GMAT PREP 670
MGMAT CAT 630
KAPLAN CAT 660

EMPOWERgmat Instructor
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 9120
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: 340 Q170 V170
Followers: 444

Kudos [?]: 2872 [1] , given: 169

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

20 May 2015, 20:17
1
KUDOS
Expert's post
Hi All,

This question can be solved with TESTing Values.

We're told that the reciprocal of R is equal to the SUM of the reciprocals of X and Y. This means….

1/R = 1/X + 1/Y

We're asked for the value of R in terms of X and Y

If X = 2 and Y = 3, then we have…

1/R = 1/2 + 1/3

1/R = 3/6 + 2/6 = 5/6

R = 6/5

So we need an answer that = 6/5 when X = 2 and Y = 3.

Answer A: XY = (2)(3) = 6 NOT a match
Answer B: X+Y = 2+3 = 5 NOT a match
Answer C: 1/(X+Y) = 1/5 NOT a match
Answer D: XY/(X+Y) = 6/5 This IS a match
Answer E: (X+Y)/XY = 5/6 NOT a match

[Reveal] Spoiler:
D

GMAT assassins aren't born, they're made,
Rich
_________________

760+: Learn What GMAT Assassins Do to Score at the Highest Levels
Contact Rich at: Rich.C@empowergmat.com

# Rich Cohen

Co-Founder & GMAT Assassin

# Special Offer: Save $75 + GMAT Club Tests 60-point improvement guarantee www.empowergmat.com/ ***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*********************** Senior Manager Joined: 23 Feb 2015 Posts: 480 Followers: 7 Kudos [?]: 140 [0], given: 161 In an electric circuit, two resistors with resistances x and [#permalink] ### Show Tags 15 Oct 2015, 23:33 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 _________________ “The heights by great men reached and kept were not attained in sudden flight but, they while their companions slept, they were toiling upwards in the night.” ― Henry Wadsworth Longfellow Last edited by iMyself on 26 Jan 2017, 08:12, edited 3 times in total. Math Forum Moderator Status: Greatness begins beyond your comfort zone Joined: 08 Dec 2013 Posts: 1168 Location: India Concentration: General Management, Strategy GPA: 3.2 WE: Information Technology (Consulting) Followers: 51 Kudos [?]: 551 [0], given: 60 Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] ### Show Tags 16 Oct 2015, 03:08 1/r = 1/x + 1/y => r = xy/(x+y) Please correct the OA provided Answer D . This question has already been discussed - in-an-electric-circuit-two-resistors-with-resistances-x-and-y-are-con-28295.html _________________ When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. - Henry Ford The Moment You Think About Giving Up, Think Of The Reason Why You Held On So Long +1 Kudos if you find this post helpful Senior Manager Joined: 23 Feb 2015 Posts: 480 Followers: 7 Kudos [?]: 140 [0], given: 161 Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] ### Show Tags 16 Oct 2015, 03:37 skywalker18 wrote: 1/r = 1/x + 1/y => r = xy/(x+y) Please correct the OA provided Answer D . This question has already been discussed - in-an-electric-circuit-two-resistors-with-resistances-x-and-y-are-con-28295.html My given answer is correct (B)....So, try for the second time! _________________ “The heights by great men reached and kept were not attained in sudden flight but, they while their companions slept, they were toiling upwards in the night.” ― Henry Wadsworth Longfellow EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 9120 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: 340 Q170 V170 Followers: 444 Kudos [?]: 2872 [0], given: 169 Re: In an electric circuit, two resistors with resistances x and [#permalink] ### Show Tags 17 Oct 2015, 10:47 Hi iMyself, Your "version" of the prompt 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 [Reveal] Spoiler: B . GMAT assassins aren't born, they're made, Rich _________________ 760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com # Rich Cohen Co-Founder & GMAT Assassin # Special Offer: Save$75 + GMAT Club Tests

60-point improvement guarantee
www.empowergmat.com/

***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************

Senior Manager
Joined: 23 Feb 2015
Posts: 480
Followers: 7

Kudos [?]: 140 [0], given: 161

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

19 Oct 2015, 11:02
EMPOWERgmatRichC wrote:
Hi iMyself,

Your "version" of the prompt 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
[Reveal] Spoiler:
B
.

GMAT assassins aren't born, they're made,
Rich

I've intentionally changed the question pattern. But, there is still something missing in your calculation or thinking.
_________________

“The heights by great men reached and kept were not attained in sudden flight but, they while their companions slept, they were toiling upwards in the night.”

Current Student
Joined: 23 Mar 2016
Posts: 37
Schools: Tulane '18 (M)
Followers: 0

Kudos [?]: 3 [0], given: 0

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

28 Apr 2016, 18:54
1/r = 1/x + 1/y
1/r = y/y(1/x)+x/x(1/y)
1/r = (y/xy) + (x/xy)
1/r = (y+x/xy)
r=(xy/x+y)
Target Test Prep Representative
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 910
Followers: 34

Kudos [?]: 510 [0], given: 5

Re: In an electric circuit, two resistors with resistances x and [#permalink]

### Show Tags

02 May 2016, 07:20
Expert's post
1
This post was
BOOKMARKED
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 sum of the reciprocals 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

Solution:

We are given the reciprocal of r is equal to the sum of the reciprocals of x and y. Thus we can say:

1/r = 1/x + 1/y

Getting a common denominator for the right side of the equation we have:

1/r = y/xy + x/xy

1/r = (y + x)/xy

If we reciprocate both sides of the equation, we have:

r = xy/(y+x)

_________________

Jeffery Miller

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

Re: In an electric circuit, two resistors with resistances x and   [#permalink] 02 May 2016, 07:20
Similar topics Replies Last post
Similar
Topics:
2 Of the two-hundred electricians who joined the Jefferson City electric 3 29 Dec 2016, 11:22
3 If two resistors, A(R1) and B(R2) stand in parallel with each other in 3 14 Jan 2016, 18:54
2 In a circuit board factory, all circuit boards that pass 2 15 Jun 2016, 16:25
2 A box contains 20 electric bulbs, out of which 4 are defective. Two 4 13 Dec 2011, 04:06
7 In an electric circuit, two resistors with resistances x and y are 9 24 Apr 2016, 03:36
Display posts from previous: Sort by