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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## Bob and Alice can finish a job together in 3 hours. tagged by: swerve ##### This topic has 3 expert replies and 0 member replies ### Top Member ## Bob and Alice can finish a job together in 3 hours. ## Timer 00:00 ## Your Answer A B C D E ## Global Stats Difficult Bob and Alice can finish a job together in 3 hours. If Bob can do the job by himself in 5 hours, what percent of the job does Alice do? A. 10 B. 20 C. 40 D. 50 E. 60 The OA is C. Can I say, 3 hours = Job/ (B + A) and 5hours = Job/B ? Then, I'm not sure Please, can anyone explain this PS question? I need help. Thanks. ### GMAT/MBA Expert Elite Legendary Member Joined 23 Jun 2013 Posted: 10111 messages Followed by: 494 members Upvotes: 2867 GMAT Score: 800 Hi swerve, We're told that Bob and Alice can finish a job together in 3 hours and that Bob can do the job by himself in 5 hours. We're asked for the percent of the job that Alice does. This is an example of a "Work Formula" question - when you have two entities working together on a task, you can use the following formula to determine how long it will take the two entities to complete the task: (A)(B)/(A+B) = time to complete the task (where A and B are the two times it takes the individuals to complete the task when working alone). Since Bob can complete the job in 5 hours, we'll set B equal to 5 and solve for A (re: Alice's time)... (A)(5)/(A+5) = 3 5A = 3A + 15 2A = 15 A = 7.5 hours to complete the job Thus, each hour... Bob does 1/5 of the job Alice does 1/7.5 = 2/15 of the job The entire job took 3 hours, so Alice completed (3)(2/15) = 6/15 = 2/5 = 40% of the job. Final Answer: C GMAT assassins aren't born, they're made, Rich _________________ Contact Rich at Rich.C@empowergmat.com ### GMAT/MBA Expert GMAT Instructor Joined 25 May 2010 Posted: 15203 messages Followed by: 1861 members Upvotes: 13060 GMAT Score: 790 swerve wrote: Bob and Alice can finish a job together in 3 hours. If Bob can do the job by himself in 5 hours, what percent of the job does Alice do? A. 10 B. 20 C. 40 D. 50 E. 60 Let the job = the LCM of the two times = 15 units. Since Bob and Alice together take 3 hours to produce the job, their combined rate = w/t = 15/3 = 5 units per hour. Since Bob on his own takes 5 hours to produce the job, Bob's rate = w/t = 15/5 = 3 units per hour. Since Bob and Alice together produce 5 units per hour, and Bob on his own produces 3 units per hour, Alice's rate = 5 - 3 = 2 units per hour. Resulting percentage for Alice = (Alice's hourly work)/(hourly work for Bob and Alice together) = 2/5 = 40%. The correct answer is C. _________________ Mitch Hunt Private Tutor for the GMAT and GRE GMATGuruNY@gmail.com If you find one of my posts helpful, please take a moment to click on the "UPVOTE" icon. Available for tutoring in NYC and long-distance. For more information, please email me at GMATGuruNY@gmail.com. Student Review #1 Student Review #2 Student Review #3 Free GMAT Practice Test How can you improve your test score if you don't know your baseline score? Take a free online practice exam. Get started on achieving your dream score today! Sign up now. ### GMAT/MBA Expert GMAT Instructor Joined 09 Apr 2015 Posted: 1461 messages Followed by: 18 members Upvotes: 39 swerve wrote: Bob and Alice can finish a job together in 3 hours. If Bob can do the job by himself in 5 hours, what percent of the job does Alice do? A. 10 B. 20 C. 40 D. 50 E. 60 Since Bob and Alice can complete a job in 3 hours, the combined rate of Bob and Alice = 1/3. Since Bob can do the job by himself in 5 hours, his rate is 1/5. Thus, the rate of Alice is 1/3 - 1/5 = 5/15 - 3/15 = 2/15. Since Alice has worked for 3 hours, her work is 3 x 2/15 = 2/5. Furthermore, since we consider the complete job the number 1, the percentage of the job done by Alice is: (2/5)/1 = 2/5 = 40% Answer: C _________________ Jeffrey Miller Head of GMAT Instruction • 1 Hour Free BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • 5-Day Free Trial 5-day free, full-access trial TTP Quant Available with Beat the GMAT members only code • Free Trial & Practice Exam BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

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