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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 ## A company has two types of machines, type R and type S. tagged by: swerve ##### This topic has 3 expert replies and 0 member replies ### Top Member ## A company has two types of machines, type R and type S. ## Timer 00:00 ## Your Answer A B C D E ## Global Stats Difficult A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hrs and a machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used? A. 3 B. 4 C. 6 D. 9 E. 12 The OA is C Source: GMAT Prep ### GMAT/MBA Expert GMAT Instructor Joined 08 Dec 2008 Posted: 13046 messages Followed by: 1253 members Upvotes: 5254 GMAT Score: 770 swerve wrote: A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hrs and a machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used? A. 3 B. 4 C. 6 D. 9 E. 12 The OA is C Source: GMAT Prep One approach is to assign a "nice" value to the job. Say, the job is to make 36 widgets. R does a certain job in 36 hours This means that machine R's rate is 1 widget/hour S does the job in 18 hours This means that machine S's rate is 2 widgets/hour So, their combined rate is 3 widgets/hour. The question asks us to complete the job in 2 hours. To make 36 widgets in 2 hours, the combined rate of the two machines must be 18 widgets/per hour. If the combined rate (of 1 R machine and 1 S machine) is 3 widgets/hour, then we'd need 6 of each machine to reach a rate of 18 widgets/hour. Answer: C Cheers, Brent _________________ Brent Hanneson â€“ Creator of GMATPrepNow.com Use my video course along with Sign up for free Question of the Day emails And check out all of these free resources GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMATâ€™s FREE 60-Day Study Guide and reach your target score in 2 months! ### GMAT/MBA Expert GMAT Instructor Joined 08 Dec 2008 Posted: 13046 messages Followed by: 1253 members Upvotes: 5254 GMAT Score: 770 swerve wrote: A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hrs and a machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used? A. 3 B. 4 C. 6 D. 9 E. 12 The OA is C Source: GMAT Prep Here's another approach . . . When it comes to questions where we must complete an entire job, I often (not always) like to know what can be accomplished in 1 unit of time (in this case, 1 hour). Machine R can complete 1/36 of the job in 1 hour. Machine S can complete 1/18 of the job in 1 hour. Since 1/36 + 1/18 = 1/12, we know that, combined, machines R and S can complete 1/12 of the job in 1 hour. From here we can apply some logic. If 1/12 of the job is completed in 1 hour (with 1 R machine and 1 S machine), then we could complete the entire job in 1 hour if we had 12 of each machine type. However, the question asks us to find the # of machines required to complete the job in 2 hours. So, we need half as many machines. In other words, we need 6 of each machine. Answer: C Cheers, Brent _________________ Brent Hanneson â€“ Creator of GMATPrepNow.com Use my video course along with Sign up for free Question of the Day emails And check out all of these free resources GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMATâ€™s FREE 60-Day Study Guide and reach your target score in 2 months! ### GMAT/MBA Expert GMAT Instructor Joined 25 Apr 2015 Posted: 2950 messages Followed by: 19 members Upvotes: 43 swerve wrote: A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hrs and a machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used? A. 3 B. 4 C. 6 D. 9 E. 12 The OA is C Source: GMAT Prep We have a combined worker problem for which we can use the following formula: work (1 machine) + work (2 machine) = total work completed Since we are completing one job, we can say: work (1 machine) + work (2 machine) = 1 We are given that a machine of type R does a certain job in 36 hours and a machine of type S does the same job in 18 hours. Thus, the rates for the two machines are as follows: rate of machine R = 1/36 rate of machine S = 1/18 We are also given that the company used the same number of each type of machine to do the job in 2 hours. If we let x = the number of each machine used, we can multiply each rate by x and we have: rate of x number of R machines = x/36 rate of x number of S machines = x/18 Finally, since we know some number of R and S machines worked for two hours, and since work = rate x time, we can calculate the work done by each type of machine. work done by x number of R machines = 2x/36 = x/18 work done by x number of S machines = 2x/18 = x/9 Now we can determine x using the combined worker formula: work (machine R) + work (machine S) = 1 x/18 + x/9 = 1 x/18 + 2x/18 = 1 3x/18 = 1 x/6 = 1 x = 6 Alternate Solution: Letâ€™s find how much time it would take to complete the job if one type R machine and one type S machine were used. In one hour, type R can complete 1/36 and type S can complete 1/18 of the job. Combined, they complete 1/36 + 1/18 = 3/36 = 1/12 of the job. If the two machines can complete 1/12 of the job in one hour, they can complete the whole job in 12 hours. Next, we notice that the job was completed in 2 hours, which is 1/6 of 12. Since time and number of machines is inversely proportional, the number of type R machine - type S machine pairs must be 6 times the number of one pair; i.e. there must be 6 such pairs. Therefore, 6 machines of type R were used. Answer: C _________________ Scott Woodbury-Stewart Founder and CEO scott@targettestprep.com See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • 1 Hour Free BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

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