NEW HERE? LEARN MORE
closeclose
Last visit was: 24 Apr 2024, 13:58 It is currently 24 Apr 2024, 13:58
Decision Tracker
My Rewards
New posts
New comers' posts

Close
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
Your Progress

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
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel

In an electric circuit, two resistors with resistances x and y are con

SORT BY:
Date
Kudos
Tags:
Difficulty: 505-555 Levelx   Algebrax                                       
Find Similar Topics 
Show Tags
Hide Tags
User avatar
Walkabout
Manager
Manager
Joined: 02 Dec 2012
Posts: 172
Own Kudos [?]: 23849 [106]
Given Kudos: 23
Send PM
In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   29 Dec 2012, 05:39
8
Kudos
96
Bookmarks
00:00
A
B
C
D
E

Difficulty:

15% (low)

Question Stats:

77% (01:04) correct 23% (01:13) wrong based on 4063 sessions
Hide Show 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
ShowHide Answer
Official Answer
D
Most Helpful Reply
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92902
Own Kudos [?]: 618803 [16]
Given Kudos: 81588
Send PM
CAT Tests Forum Quiz Mode
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   29 Dec 2012, 05:42
6
Kudos
10
Bookmarks
Expert Reply
Walkabout 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).

Answer: D.
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92902
Own Kudos [?]: 618803 [7]
Given Kudos: 81588
Send PM
CAT Tests Forum Quiz Mode
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   28 Dec 2013, 03:42
4
Kudos
3
Bookmarks
Expert Reply
aeglorre wrote:
Bunuel wrote:
Walkabout 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).

Answer: D.



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.
General Discussion
User avatar
DaVagabond
Intern
Intern
Joined: 03 Nov 2009
Posts: 47
Own Kudos [?]: 57 [5]
Given Kudos: 18
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   29 Dec 2012, 05:54
3
Kudos
2
Bookmarks
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
User avatar
aeglorre
Manager
Manager
Joined: 12 Jan 2013
Posts: 107
Own Kudos [?]: 206 [1]
Given Kudos: 47
Send PM
GMAT ToolKit User
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   27 Dec 2013, 10:31
1
Kudos
Bunuel wrote:
Walkabout 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).

Answer: D.



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?
User avatar
L
EMPOWERgmatRichC
GMAT Club Legend
GMAT Club Legend
Joined: 19 Dec 2014
Status:GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Posts: 21846
Own Kudos [?]: 11665 [2]
Given Kudos: 450
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Send PM
Re: In an electric circuit, two resistors with resistances x and y are [#permalink] New post   13 Mar 2015, 12:18
2
Bookmarks
Expert Reply
Hi All,

While this question looks a little "crazy", it 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.

The only answer that matches is
Show Spoiler
D


GMAT assassins aren't born, they're made,
Rich
User avatar
L
EMPOWERgmatRichC
GMAT Club Legend
GMAT Club Legend
Joined: 19 Dec 2014
Status:GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Posts: 21846
Own Kudos [?]: 11665 [3]
Given Kudos: 450
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   20 May 2015, 20:17
2
Kudos
1
Bookmarks
Expert Reply
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

Final Answer:
Show Spoiler
D


GMAT assassins aren't born, they're made,
Rich
User avatar
P
JeffTargetTestPrep
Target Test Prep Representative
Joined: 04 Mar 2011
Status:Head GMAT Instructor
Affiliations: Target Test Prep
Posts: 3043
Own Kudos [?]: 6272 [3]
Given Kudos: 1646
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   02 May 2016, 07:20
1
Kudos
2
Bookmarks
Expert Reply
Walkabout 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


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)

The answer is D.
avatar
G
Shiv2016
Director
Director
Joined: 02 Sep 2016
Posts: 528
Own Kudos [?]: 194 [0]
Given Kudos: 275
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   04 Sep 2017, 03:23
Bunuel

Why are we not considering r= x+y as it is mentioned that r is the combined resistance? Is it because we don't know how they are connected.
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92902
Own Kudos [?]: 618803 [0]
Given Kudos: 81588
Send PM
CAT Tests Forum Quiz Mode
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   04 Sep 2017, 03:27
Expert Reply
Shiv2016 wrote:
Bunuel

Why are we not considering r= x+y as it is mentioned that r is the combined resistance? Is it because we don't know how they are connected.


The question clearly gives the relationship between r, x, and y and it is NOT r = x + y: the reciprocal of r is equal to the sum of the reciprocals of x and y.
User avatar
L
dave13
VP
VP
Joined: 09 Mar 2016
Posts: 1160
Own Kudos [?]: 1017 [0]
Given Kudos: 3851
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   07 Jan 2018, 06:05
Bunuel wrote:
Walkabout 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).

Answer: D.


Bunuel as per this wording "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" i did this 1/r = 1/x + 1/y and chose C

can you explan how you got this ? --> r=xy/(x + y)

thanks !
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92902
Own Kudos [?]: 618803 [1]
Given Kudos: 81588
Send PM
CAT Tests Forum Quiz Mode
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   07 Jan 2018, 06:10
1
Bookmarks
Expert Reply
dave13 wrote:
Bunuel wrote:
Walkabout 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).

Answer: D.


Bunuel as per this wording "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" i did this 1/r = 1/x + 1/y and chose C

can you explan how you got this ? --> r=xy/(x + y)

thanks !


It's very simple algebraic manipulation.

1/r = 1/x + 1/y

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

xy = r*(x + y)

r = xy/(x + y)
avatar
B
HiItsAlif
Intern
Intern
Joined: 20 Feb 2018
Posts: 1
Own Kudos [?]: 0 [0]
Given Kudos: 0
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   27 Mar 2018, 09:43
Hi all,

Can anyone advise why its not good logic to flip the reciprocals as the first step?
User avatar
L
EMPOWERgmatRichC
GMAT Club Legend
GMAT Club Legend
Joined: 19 Dec 2014
Status:GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Posts: 21846
Own Kudos [?]: 11665 [2]
Given Kudos: 450
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   27 Mar 2018, 10:52
1
Kudos
1
Bookmarks
Expert Reply
HiItsAlif wrote:
Hi all,

Can anyone advise why its not good logic to flip the reciprocals as the first step?


Hi HiItsAlif,

The 'math step' that you're thinking about is NOT mathematically correct. For example:

1/2 = 1/3 + 1/6

However, if you 'flip' the reciprocals, you end up with...

2 = 3 + 6

...which is not correct.

GMAT assassins aren't born, they're made,
Rich
User avatar
D
avigutman
Tutor
Joined: 17 Jul 2019
Posts: 1304
Own Kudos [?]: 2285 [3]
Given Kudos: 66
Location: Canada
Schools: NYU Stern (WA)
GMAT 1: 780 Q51 V45
GMAT 2: 780 Q50 V47
GMAT 3: 770 Q50 V45
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   31 Dec 2020, 09:37
2
Kudos
1
Bookmarks
Expert Reply
Video solution from Quant Reasoning:
Subscribe for more: https://www.youtube.com/QuantReasoning? ... irmation=1
User avatar
B
Thib33600
Intern
Intern
Joined: 08 Feb 2023
Posts: 33
Own Kudos [?]: 1 [0]
Given Kudos: 22
Send PM
Re: In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   03 Feb 2024, 13:03
Bunuel wrote:
Walkabout 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).

Answer: D.


Why dowe flip the side at the end ?
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92902
Own Kudos [?]: 618803 [0]
Given Kudos: 81588
Send PM
CAT Tests Forum Quiz Mode
In an electric circuit, two resistors with resistances x and y are con [#permalink] New post   03 Feb 2024, 13:14
Expert Reply
Thib33600 wrote:
Bunuel wrote:
Walkabout 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).

Answer: D.


Why dowe flip the side at the end ?


    1/r = 1/x + 1/y

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

    r = xy/(y + x)

Also, explained HERE.
GMAT Club Bot
gmatclubot
In an electric circuit, two resistors with resistances x and y are con [#permalink]
03 Feb 2024, 13:14
Moderators:
Bunuel
Math Expert
92902 posts
yashikaaggarwal
Senior Moderator - Masters Forum
3137 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne
Main navigation
GMAT Resources
Partners
MBA Resources
www.gmac.com|www.mba.com | | GMAT Club Rules | Terms and Conditions