tarek99 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 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

In one hour, R can complete 1/36 of job and S can complete 1/18 of job

So, working together (one R and one S) for one hour, job completed = 1/36+1/18 = 3/36=1/12

Working for 2 hours (one R and one S), job completed = 2/12

So if you wanna complete the whole work in 2 hours, no. of R and S required = 12/2 =6

(This means we need 6Rs and 6Ss)

answer C