GMAT Prep Question (work)

[gxj007]

Can anyone, explain the how to get the answer for this one? thanks!

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 machine S does the same job in 18 hours. If the company used the same number of each 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


answer:
c) 6

[ankitns]

The rate at which R does the work is 1/36.
The rate at which S does the work is 1/18.

If they work together, the combined rate will be
= 1/36 + 1/18
= 3/36
= 1/12

To finish the work in 2 hours the combined rate should be 1/2. So think of it in this way: Which number multiplied by the rate of 1/12 will give you the desired rate of 1/2.

N * (1/12) = 1/2

Solving this we will get N = 6.


Hope this helps.

Can anyone, explain the how to get the answer for this one? thanks!

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 machine S does the same job in 18 hours. If the company used the same number of each 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


answer:
c) 6

[ghacker]

The answer is 6

assume the the job is making toys and there are 72 toys since R does the job in 36 hours , per hour R makes 2 toys

Similar since S does it in 18 hours , per hour S makes 4 toys

Both together makes 72 toys in 2 hours , so per hour they make 36 toys

But the factory uses the same number of machines

Then 36 = 6X number of machines

Number of machines = 6