Live GMAT classes with Target Test Prep start Nov 21 and Nov 22. First class free.

See classes
Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

Newsweek Readers’ Choice Awards 2026 winner: Target Test Prep, #1 Test Prep

Target Test Prep was voted #1 Best Test Prep in the 2026 Newsweek Readers’ Choice™ Awards.

★★★★★5.0563 reviews
GMATLiveTeachStarts Nov 22
Josh Braslow, Target Test Prep GMAT expert
LIVE ONLINE CLASSES

Get Ready for GMAT Test Day Faster with Live Online Classes

with Josh Braslow, 20+ years in test prep

Nov 22 · Josh Braslow
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes + 6 months of TTP OnDemand
  • Attend the first class for free
  • Every class is recorded, so you never fall behind
View classes & enroll
Limited seats availableTarget Test Prep
EALiveTeachOnDemandStart anytime
EXECUTIVE ASSESSMENT

Target Test Prep EA OnDemand

Self-paced EA prep. Study on your schedule.

EXECUTIVE ASSESSMENT

with

165+ EA score guarantee
$05-day trial no automatic billing
Schedule
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll Start free 5-day trial
Limited cohort · enrollment openTrial includes full course accessTarget Test Prep
GMATOnDemandStart anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 563 reviewsTarget Test Prep

One robot builds a robot in 1 hour, another one builds a

Expert replies
Post new topic Post Reply
Subscribe topic
Previous Topic Next Topic
Legendary Member
Posts: 2276
Joined: Sat Oct 14, 2017 6:10 amFollowed by: 3 members
by VJesus12 » Mon May 28, 2018 2:01 am
Timer
00:00
Your answer
Difficulty–

right wrong

Pick an answer to see how members did

One robot builds a robot in 1 hour, another one builds a robot in 2 hours. The 2 robots work together and when a new robot is complete, it joins the others, working at a constant rate of one robot in 2 hours. How much time will it take until there are 5 robots altogether, if the robots build only one robot at a time?

A. 70 min
B. 94 min
C. 110 min
D. 112 min
E. 120 min

The OA is the option B.

This is a really nice question, but I couldn't solve it. I got confused with the rates. Could anyone give me some help? Please.
Top
Join the discussion
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members
Location: New York, NY
Thanked: 13060 times
GMAT Score:790

by GMATGuruNY » Mon May 28, 2018 2:54 am
VJesus12 wrote:One robot builds a robot in 1 hour, another one builds a robot in 2 hours. The 2 robots work together and when a new robot is complete, it joins the others, working at a constant rate of one robot in 2 hours. How much time will it take until there are 5 robots altogether, if the robots build only one robot at a time?

A. 70 min
B. 94 min
C. 110 min
D. 112 min
E. 120 min
Let each robot = 2 units.
Since a faster robot takes 1 hour to produce a 2-unit robot, the rate for a faster robot = w/t = 2/1 = 2 units per hour.
Since a slower robot takes 1 hour to produce a 2-unit robot, the rate for a slower robot = w/t = 2/2 = 1 unit per hour.
Each additional robot produced takes 2 hours to produce a 2-unit robot.
Thus, each additional robot produced is a SLOWER ROBOT with a rate of 1 unit per hour.

Time for the first 2 robots to produce a third 2-unit robot = w/(combined rate for 1 faster and 1 slower robot) = 2/(2+1) = 2/3 hour = 40 minutes.
Time for the resulting 3 robots to produce a fourth 2-unit robot = w/(combined rate for 1 faster and 2 slower robots) = 2/(2+1+1) = 1/2 hour = 30 minutes.
Time for the resulting 4 robots to produce a fifth 2-unit robot = w/(combined rate for 1 faster and 3 slower robots) = 2/(2+1+1+1) = 2/5 hour = 24 minutes.
Total time for there to be 5 robots = 40+30+24 = 94 minutes.

The correct answer is B.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Join the discussion
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Mon May 28, 2018 10:55 pm

by katrinamonarco » Mon May 28, 2018 11:00 pm
94 min

https://www.agftutoring.com/tests/gmat/
Top
Join the discussion
User avatar
GMAT Instructor
Posts: 1462
Joined: Thu Apr 09, 2015 9:34 amFollowed by: 22 members
Location: New York, NY
Thanked: 39 times

by Jeff@TargetTestPrep » Wed May 30, 2018 4:19 pm
VJesus12 wrote:One robot builds a robot in 1 hour, another one builds a robot in 2 hours. The 2 robots work together and when a new robot is complete, it joins the others, working at a constant rate of one robot in 2 hours. How much time will it take until there are 5 robots altogether, if the robots build only one robot at a time?

A. 70 min
B. 94 min
C. 110 min
D. 112 min
E. 120 min
The rate of the first robot is 1 and the rate of the second robot is 1/2. Thus it will take both of them 1/(1 + 1/2) = 1/(3/2) = 2/3 hour = 40 minutes to build a new robot.

Now we have 3 robots. Since the rate of this new robot (or any new robot) is 1/2, it will take the three of them 1/(1 + 1/2 + 1/2) = 1/2 hour = 30 minutes to build a new robot.

Now we have 4 robots and it will take the four of them 1/(1 + 1/2 + 1/2 + 1/2) = 1/(5/2) = 2/5 hour = 24 minutes to build a new robot.

Since now we have 5 robots, we see that it takes 40 + 30 + 24 = 94 minutes to have 5 (2 existing and 3 built) robots.

Answer: B

Jeffrey Miller
Head of GMAT Instruction
[email protected]

Image

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews
Top
Join the discussion