GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 23 May 2019, 10:59

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.

Close

Request Expert Reply

Confirm Cancel

In the formula V = 1/(2r)^3, if r is halved, then V is multi

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

 
Senior Manager
Senior Manager
avatar
Joined: 10 Mar 2008
Posts: 302
In the formula V = 1/(2r)^3, if r is halved, then V is multi  [#permalink]

Show Tags

New post Updated on: 21 Feb 2014, 05:10
1
12
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

72% (01:05) correct 28% (01:15) wrong based on 471 sessions

HideShow timer Statistics

In the formula V = 1/(2r)^3, if r is halved, then V is multiplied by

A. 64
B. 8
C. 1
D. 1/8
E. 1/64

Originally posted by vksunder on 15 Jul 2008, 12:39.
Last edited by Bunuel on 21 Feb 2014, 05:10, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
Most Helpful Expert Reply
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 55266
Re: In the formula V= 1/(2r)^3, if r is halved, then V is  [#permalink]

Show Tags

New post 21 Feb 2014, 05:28
2
6
benjamin09 wrote:
Can someone solve this question with an explanation?

Thanks


In the formula V = 1/(2r)^3, if r is halved, then V is multiplied by

A. 64
B. 8
C. 1
D. 1/8
E. 1/64

Given: \(V = \frac{1}{(2r)^3}=\frac{1}{8r^3}\)

Now, halved r is r/2, substitute r/2 instead of r: \(V' = \frac{1}{(2*\frac{r}{2})^3}=\frac{1}{r^3}\).

\(V =\frac{1}{8r^3}\) should be multiplied by 8 to get \(V'=\frac{1}{r^3}\).

Answer: B.

Or:
When r=2, then \(V = \frac{1}{(2r)^3}=\frac{1}{64}\).
When r=1, then \(V' = \frac{1}{(2r)^3}=\frac{1}{8}\).

1/64 should be multiplied by 8 to get 1/8.

Answer: B.

Similar question to practice: if-the-function-q-is-defined-by-the-formula-q-5w-4x-z-131051.html

Hope it helps.
_________________
General Discussion
SVP
SVP
User avatar
Joined: 30 Apr 2008
Posts: 1801
Location: Oklahoma City
Schools: Hard Knocks
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 12:42
B - about had that backwards :( :shock:

Make r = 1/2, V = 1

Make r = 1, v = 1/8
_________________
------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

GMAT Club Premium Membership - big benefits and savings
Senior Manager
Senior Manager
avatar
Joined: 07 Jan 2008
Posts: 250
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 13:00
I always mess up these kinds...
Say r=2 => V1 = 1/64
when r=1; V2 = 1/8
V2 = 8*V1.
B?
SVP
SVP
User avatar
Joined: 30 Apr 2008
Posts: 1801
Location: Oklahoma City
Schools: Hard Knocks
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 13:05
I totally agree with you. I mess them up to and about did this one. I saw that if I make 2r = 1, then r=1/2 so to the power of 3 doesn't matter, it's still 1/1. Then I did r =1 (double it) so then V = 1/8. So I was thinking oh, go from 1 to 1/8 multiply by 1/8...but I had worked the problem backwards. There are tricks the authors throw at us, and then there are tricks we bring to the test ourselves. We have to eliminate our own tricks. Easier said than done!

mbawaters wrote:
I always mess up these kinds...
Say r=2 => V1 = 1/64
when r=1; V2 = 1/8
V2 = 8*V1.
B?

_________________
------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

GMAT Club Premium Membership - big benefits and savings
Senior Manager
Senior Manager
avatar
Joined: 10 Mar 2008
Posts: 302
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 13:09
So isnt the qtn essentially testing us on: if r is halved then what should V be multiplied by to retain the intial formula???
Senior Manager
Senior Manager
avatar
Joined: 07 Jan 2008
Posts: 250
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 15:30
vksunder wrote:
So isnt the qtn essentially testing us on: if r is halved then what should V be multiplied by to retain the intial formula???

I think its asking us to reflect the changes in the value of V if r is halved!
Senior Manager
Senior Manager
User avatar
Joined: 09 Oct 2007
Posts: 438
Re: PS: formula V= 1/(2r)^3  [#permalink]

Show Tags

New post 15 Jul 2008, 23:22
vksunder wrote:
So isnt the qtn essentially testing us on: if r is halved then what should V be multiplied by to retain the intial formula???


Not really, it's asking what happens to V when r is halved. In this type of problems, just pick an easy value for r and solve both equations.
Intern
Intern
avatar
Joined: 29 Jan 2014
Posts: 17
GMAT 1: 510 Q36 V26
GMAT 2: 550 Q38 V28
Re: In the formula V= 1/(2r)^3, if r is halved, then V is  [#permalink]

Show Tags

New post 21 Feb 2014, 03:14
Can someone solve this question with an explanation?

Thanks
_________________
Please give me Kudos if you like my post. Thank you
Intern
Intern
avatar
Joined: 29 Jan 2014
Posts: 17
GMAT 1: 510 Q36 V26
GMAT 2: 550 Q38 V28
Re: In the formula V = 1/(2r)^3, if r is halved, then V is multi  [#permalink]

Show Tags

New post 21 Feb 2014, 07:34
Thanks a lot Bunuel!
_________________
Please give me Kudos if you like my post. Thank you
EMPOWERgmat Instructor
User avatar
V
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 14197
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Re: In the formula V = 1/(2r)^3, if r is halved, then V is multi  [#permalink]

Show Tags

New post 19 Mar 2018, 10:54
Hi All,

This question is perfect for TESTing Values.

IF... R=2
V = (1/(2x2)^3 = 1/64

Halving R gives us R=1...
V = (1/2x1)^3 = 1/8

1/8 = 8/64...
so going from 1/64 to 8/64 means that V increased 8 times.

Final Answer:

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

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

Rich Cohen

Co-Founder & GMAT Assassin

Special Offer: Save $75 + GMAT Club Tests Free
  Official GMAT Exam Packs + 70 Pt. Improvement Guarantee
www.empowergmat.com/
VP
VP
User avatar
D
Joined: 09 Mar 2016
Posts: 1284
Re: In the formula V = 1/(2r)^3, if r is halved, then V is multi  [#permalink]

Show Tags

New post 19 Mar 2018, 12:49
Bunuel wrote:
benjamin09 wrote:
Can someone solve this question with an explanation?

Thanks


In the formula V = 1/(2r)^3, if r is halved, then V is multiplied by

A. 64
B. 8
C. 1
D. 1/8
E. 1/64

Given: \(V = \frac{1}{(2r)^3}=\frac{1}{8r^3}\)

Now, halved r is r/2, substitute r/2 instead of r: \(V' = \frac{1}{(2*\frac{r}{2})^3}=\frac{1}{r^3}\).

\(V =\frac{1}{8r^3}\) should be multiplied by 8 to get \(V'=\frac{1}{r^3}\).

Answer: B.

Or:
When r=2, then \(V = \frac{1}{(2r)^3}=\frac{1}{64}\).
When r=1, then \(V' = \frac{1}{(2r)^3}=\frac{1}{8}\).

1/64 should be multiplied by 8 to get 1/8.

Answer: B.

Similar question to practice: http://gmatclub.com/forum/if-the-functi ... 31051.html

Hope it helps.


why answer is B amd not D ? :?
Target Test Prep Representative
User avatar
G
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 2823
Re: In the formula V = 1/(2r)^3, if r is halved, then V is multi  [#permalink]

Show Tags

New post 05 Apr 2018, 11:11
vksunder wrote:
In the formula V = 1/(2r)^3, if r is halved, then V is multiplied by

A. 64
B. 8
C. 1
D. 1/8
E. 1/64


Since (2r)^3 = 8r^3, we can rewrite the formula as V = 1/(8r^3).

Let’s substitute r/2 instead of r in the first equation: V = 1/(2(r/2))^3 = 1/r^3 = 8*1/(8r^3).

As we can see, when we halve r, V gets multiplied by 8.

Alternate Solution:

Let’s let the original value of r = 2. Substituting 2 for r into the original equation yields:

Original V = 1/[(2)(2)]^3,

Original V = 1/(4)^3

Original V = 1/64

Now we halve r, so it is now equal to 1. Substituting 1 for r gives us:

New V = 1/[(2)(1)]^3

New V = 1/ 8

We see that the original value of V (1/64), when multiplied by 8, gives us the new value of V:

(1/64) x 8 = 8/64 = 1/8

Answer: B
_________________

Jeffrey Miller

Head of GMAT Instruction

Jeff@TargetTestPrep.com
TTP - Target Test Prep Logo
122 Reviews

5-star rated online GMAT quant
self study course

See why Target Test Prep is the top rated GMAT quant course on GMAT Club. Read Our Reviews

If you find one of my posts helpful, please take a moment to click on the "Kudos" button.

GMAT Club Bot
Re: In the formula V = 1/(2r)^3, if r is halved, then V is multi   [#permalink] 05 Apr 2018, 11:11
Display posts from previous: Sort by

In the formula V = 1/(2r)^3, if r is halved, then V is multi

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  


Copyright

GMAT Club MBA Forum Home| About| Terms and Conditions and Privacy Policy| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.