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 hrs 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 12 hours, how many machines of type R were used?	Choose the correct choice from the following answers:
A) 1
B) 4
C) 6
D) 9
E) 12
Rate of machine R =1/36   Rate of machine S =1/18   since same no of machines used for R and S to do the same work in 12 hrs   So collective rate needed to finish the work in 12 hrs= 1/12   Let the no of machine be x   So, x/36 +x/18 =1/12   x3/36=1/12   x=1   So no of machine R is 1   1nswer 1
<result>A</result>
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 hrs 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 12 hours, how many machines of type R were used?