• NEW! FREE Beat The GMAT Quizzes
    NEW! FREE Beat The GMAT Quizzes
    NEW! FREE Beat The GMAT Quizzes
    Hundreds of Questions Highly Detailed Reporting Expert Explanations TAKE A FREE GMAT QUIZ
  • 7 CATs FREE!
    If you earn 100 Forum Points

    Engage in the Beat The GMAT forums to earn
    100 points for $49 worth of Veritas practice GMATs FREE

    Veritas Prep
    VERITAS PRACTICE GMAT EXAMS
    Earn 10 Points Per Post
    Earn 10 Points Per Thanks
    Earn 10 Points Per Upvote
    REDEEM NOW

If Robert can assemble a model car in 30 minutes...

This topic has 4 expert replies and 0 member replies

If Robert can assemble a model car in 30 minutes...

Post
If Robert can assemble a model car in 30 minutes and Craig can assemble the same model car in 20 minutes, how long would it take tehm, working together, to assemble the model car?

A. 12 minutes
B. 13 minutes
C. 14 minutes
D. 15 minutes
E. 16 minutes

The OA is A.

I'm confused by this PS question. Experts, any suggestion? Thanks in advance.

How can I determine the rate of each one? Can I say that the model car has a n number of pieces?

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Legendary Member
Joined
20 Jul 2017
Posted:
503 messages
Followed by:
11 members
Upvotes:
86
GMAT Score:
770
Top Reply
Post
The easiest approach on this question is to use the combined work equation $$\frac{1}{A}+\frac{1}{B}=\frac{1}{A and B}$$ Where A and B are the time it would take each person (or machine, pump, printer, etc. depending on the question) to finish a job alone, and A and B is the time it would take both to finish the same job together.

In this problem, A will be the time it takes Robert to assemble the model car alone, B will be the time it takes Craig to assemble the model car alone, and A and B will be the time it takes the two of them to assemble the model car together. Plugging in the information from our equation gives: $$\frac{1}{20}+\frac{1}{30}=\frac{1}{A\ and\ B}$$ Then solving for A and B gives $$\frac{3}{60}+\frac{2}{60}=\frac{1}{A\ and\ B}$$ $$\frac{5}{60}=\frac{1}{A\ and\ B}$$ $$5\left(A\ and\ B\right)=60$$ $$A\ and\ B=12$$ So Robert and Craig can assemble the model car together in 12 minutes, and the correct answer is A.

_________________


Erika John - Content Manager/Lead Instructor
https://gmat.prepscholar.com/gmat/s/

Get tutoring from me or another PrepScholar GMAT expert: https://gmat.prepscholar.com/gmat/s/tutoring/

Learn about our exclusive savings for BTG members (up to 25% off) and our 5 day free trial

Check out our PrepScholar GMAT YouTube channel, and read our expert guides on the PrepScholar GMAT blog

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Legendary Member
Joined
14 Jan 2015
Posted:
2667 messages
Followed by:
122 members
Upvotes:
1153
GMAT Score:
770
Post
LUANDATO wrote:
If Robert can assemble a model car in 30 minutes and Craig can assemble the same model car in 20 minutes, how long would it take tehm, working together, to assemble the model car?

A. 12 minutes
B. 13 minutes
C. 14 minutes
D. 15 minutes
E. 16 minutes

The OA is A.

I'm confused by this PS question. Experts, any suggestion? Thanks in advance.

How can I determine the rate of each one? Can I say that the model car has a n number of pieces?
If 'A' is the amount of time for one entity to complete a job, and 'B' is the amount of time for a second entity to complete the same job, then the time for the two to complete the job together is (A*B)/(A + B)

A= 30 minutes
B= 20 minutes

A*B/(A+B) = 30*20/(30+20) = 30*20/50 = 30*2/5 = 60/5 = 12. The answer is A

_________________
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course

  • +1 Upvote Post
  • Quote
  • Flag
Enroll in a Veritas Prep GMAT class completely for FREE. Wondering if a GMAT course is right for you? Attend the first class session of an actual GMAT course, either in-person or live online, and see for yourself why so many students choose to work with Veritas Prep. Find a class now!

GMAT/MBA Expert

Legendary Member
Joined
14 Jan 2015
Posted:
2667 messages
Followed by:
122 members
Upvotes:
1153
GMAT Score:
770
Post
LUANDATO wrote:
If Robert can assemble a model car in 30 minutes and Craig can assemble the same model car in 20 minutes, how long would it take tehm, working together, to assemble the model car?

A. 12 minutes
B. 13 minutes
C. 14 minutes
D. 15 minutes
E. 16 minutes

The OA is A.

I'm confused by this PS question. Experts, any suggestion? Thanks in advance.

How can I determine the rate of each one? Can I say that the model car has a n number of pieces?
Note that the formula (A*B)/(A + B) = T is really the same one that Erika used. If the first rate is 1/A and the second rate is 1/B the combined rate is 1/A + 1/B, which comes out to (A+B)/A*B. Because rate and time have a reciprocal relationship, a rate of (A+B)/A*B is equivalent to a time of A*B/(A + B)

_________________
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course

  • +1 Upvote Post
  • Quote
  • Flag
Enroll in a Veritas Prep GMAT class completely for FREE. Wondering if a GMAT course is right for you? Attend the first class session of an actual GMAT course, either in-person or live online, and see for yourself why so many students choose to work with Veritas Prep. Find a class now!
Post
BTGmoderatorLU wrote:
If Robert can assemble a model car in 30 minutes and Craig can assemble the same model car in 20 minutes, how long would it take tehm, working together, to assemble the model car?

A. 12 minutes
B. 13 minutes
C. 14 minutes
D. 15 minutes
E. 16 minutes

The combined rate of Robert and Craig is 1/20 + 1/30 = 3/60 + 2/60 = 5/60. We can interpret this rate as “They can assemble 5 model cars in 60 minutes.” Since time is the reciprocal of rate, it would take them 60/5 = 12 minutes to complete one car.

Answer: A

  • +1 Upvote Post
  • Quote
  • Flag
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • The Princeton Review
    FREE GMAT Exam
    Know how you'd score today for $0

    Available with Beat the GMAT members only code

    MORE DETAILS
    The Princeton Review

Top First Responders*

1 Brent@GMATPrepNow 41 first replies
2 Ian Stewart 37 first replies
3 Jay@ManhattanReview 32 first replies
4 GMATGuruNY 26 first replies
5 Scott@TargetTestPrep 14 first replies
* Only counts replies to topics started in last 30 days
See More Top Beat The GMAT Members

Most Active Experts

1 image description Scott@TargetTestPrep

Target Test Prep

199 posts
2 image description Max@Math Revolution

Math Revolution

92 posts
3 image description Brent@GMATPrepNow

GMAT Prep Now Teacher

74 posts
4 image description GMATGuruNY

The Princeton Review Teacher

45 posts
5 image description Ian Stewart

GMATiX Teacher

43 posts
See More Top Beat The GMAT Experts