BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

Functions

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 2326
Joined: Mon Jul 28, 2008 3:54 amFollowed by: 2 members
Thanked: 173 times
GMAT Score:710
by gmatmachoman » Tue May 18, 2010 4:12 am
The cost C per km. of electric cable is given by C = 120 /x+ 600 x, where x is the cross - section in square cms. Find the cross section for which the cost is least and also find the least cost per km.?


OA: 240 sqrt (5)
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 3650
Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members
Location: India
Thanked: 267 times
GMAT Score:760

by sanju09 » Tue May 18, 2010 6:05 am
gmatmachoman wrote:The cost C per km. of electric cable is given by C = 120 /x+ 600 x, where x is the cross - section in square cms. Find the cross section for which the cost is least and also find the least cost per km.?


OA: 240 sqrt (5)
NOT FOR GMAT

Kamaal ho tussi...

C' = -120/x^2 + 600

For minimum C, C' = 0

Or 120/x^2 = 600

Or x^2 = 1/5

(C'' for verification is skipped here)

Or the cross section for which the cost is least is [spoiler]1/√5[/spoiler], and hence the minimum cost

= 120 √5 + 600/√5

= 1200/√5

= [spoiler]240 √5[/spoiler].
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 90
Joined: Thu Jan 14, 2010 5:42 am
Thanked: 2 times

by dkumar.83 » Tue May 18, 2010 8:34 am
Hi, can u elaborate wat does the cross section means?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 537
Joined: Fri Jan 22, 2010 10:06 pmFollowed by: 1 member
Thanked: 14 times

by frank1 » Tue May 18, 2010 8:59 pm
dkumar.83 wrote:Hi, can u elaborate wat does the cross section means?
excatly this was the first question that came to my mind when i read this question......lol
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 265
Joined: Mon Dec 28, 2009 9:45 pmFollowed by: 2 members
Thanked: 26 times
GMAT Score:760

by mj78ind » Tue May 18, 2010 9:54 pm
Cross section is if you were to cut the wire and hold it in front of you with the wire pointing towards you ..... you could see a circle, square etc. - refer link - https://www.google.co.in/imgres?imgurl=h ... CB4Q9QEwAA

but it does not matter what is the shape since the question states the area is "x" sq cm....
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 576
Joined: Sat Mar 13, 2010 8:31 pmFollowed by: 1 member
Thanked: 97 times

by liferocks » Tue May 18, 2010 10:05 pm
Let a=120 /x and b=600 x
then ab=120/x*600x=120*600..a constant

if ab is a constant a+b will be minimum when a=b

so when cost is minimum, a=b=sqrt(120*600) or 120*sqrt(5)

so for minimum cost 600x=120*sqrt(5) or x=1/sqrt(5)

and minimum cost is [spoiler]2*120*sqrt(5)=240*sqrt(5)[/spoiler]

@Sanju,Doesn't take much time to calculate..why do you think its not a GMAT type?
"If you don't know where you are going, any road will get you there."
Lewis Carroll
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 3650
Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members
Location: India
Thanked: 267 times
GMAT Score:760

by sanju09 » Wed May 19, 2010 1:57 am
liferocks wrote:Let a=120 /x and b=600 x
then ab=120/x*600x=120*600..a constant

if ab is a constant a+b will be minimum when a=b

so when cost is minimum, a=b=sqrt(120*600) or 120*sqrt(5)

so for minimum cost 600x=120*sqrt(5) or x=1/sqrt(5)

and minimum cost is [spoiler]2*120*sqrt(5)=240*sqrt(5)[/spoiler]

@Sanju,Doesn't take much time to calculate..why do you think its not a GMAT type?
Didn't think that way on this occasion, otherwise the Maxima and Minima concepts of calculus are not checked on GMAT, and I did it that way only, that's why the undue remark. Thanks
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1