GMAT PREP????

dferm · Posted: Mon Mar 24, 2008 1:38 pm Post subject: GMAT PREP????

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 hours 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

xilef · Posted: Mon Mar 24, 2008 1:58 pm Post subject:

using work formula to find the time it takes for both machines to do the same work:

(18*36)/54=12 hrs for both machines to complete the job

2 hrs is 6 times faster so the company needs 6 of each to do it in 12hr.

Answer C.

preciousrain7 · Posted: Mon Mar 24, 2008 2:47 pm Post subject:

xilef · Posted: Mon Mar 24, 2008 3:04 pm Post subject:

It takes both working together (one of each machine) to do the job in 12hrs. The company used the same number of each type of machine to do the job in 2 hours, which is 6 times faster than the the time it takes one of each machine to do the job, therefore, company used 6 of each.

preciousrain7 · Posted: Mon Mar 24, 2008 3:31 pm Post subject:

daduck112 · Posted: Mon Jul 07, 2008 1:09 pm Post subject:

another way to look at things...

R does 1/36 the job

S does 1/18 the job

R+S = 1/12 the job or 12 hours to do 1 job

you want to do it in 2 hours....

so you you'll 12/2 = {6 machines each.}

AleksandrM · Posted: Mon Jul 07, 2008 4:05 pm Post subject:

1/36 + 1/18 = 1/12 so together they can do 1 job in 12 hours. Then, I set up the following equation:

2/1/12 = x/1/2 [translation: if 2 machines complete 1 job in 12, then how many machines will complete 1 job in 2 hours?]

24 = 2x

x = 12 machines, 6 of each type.