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.

gxj007 wrote: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