GMAT Prep #1 - Machines type R and Type S

[veenitj]

A compnay has two types of M/c, type R and type S. Operating at a constant rate, am/c of type R does a certain job in 36 hr and m/c type S does the sam job in 18 hr. If the copmany used the same # of each type to do this job in 2 hr, how many m/c of type R wer used?

3
4
6
9
12

We can easly cross out option a,b,d. I did it correct but some how I am not able to solve the problem.

Will appreciate any replies.

Thanks
VJ

OA=c

[Bill@VeritasPrep]

Each type R has a rate of 1/36, and each type S has a rate of 1/18 (or 2/36). This means that each type S machine does twice as much work as each type R machine.

In order to complete the job in 2 hours, the combined rate of all machines must be 1/2 (or 18/36). Since each type S machine does twice as much work as each type R machine, then the total amount of work done by type S machines must be twice as much as the total amount of work done by type R machines.

Of the combined rate of 18/36, 12/36 must be type S and 6/36 must be type R.

Type S: 12/36 divided by the individual rate of 2/36 means that 6 type S machines were used.

Type R: 6/36 divided by the individual rate of 1/36 means that 6 type R machines were used.

Since the question asks about type R, the correct answer is C.

[niketdoshi123]

A compnay has two types of M/c, type R and type S. Operating at a constant rate, am/c of type R does a certain job in 36 hr and m/c type S does the sam job in 18 hr. If the copmany used the same # of each type to do this job in 2 hr, how many m/c of type R wer used?

3
4
6
9
12

# of machines used of each type = n
Rate of R = 1/36
Combined rate of all R machines= n/36
Rate of S = 1/18
Combined rate of all S machines= n/18

Combined Rate of all machines= 1/2
(n/36)+(n/18)=1/2
3*n/36=1/2
n=6
hence answer is C[/quote]

[GMATGuruNY]

To avoid fractions, plug in a value for the job. I posted a solution here:

https://www.beatthegmat.com/gmat-prep-1- ... 08621.html

[mdavidm_531]

Hi, Experts,

I see that this assumption is very important to the problem: "If the copmany used the same # of each type to do this job"

I wonder what will happen if such assumption is not stated. Would we need to have two equations?

Thanks