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 1 hours, how many machines of type R were used? Select.
A) 3  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 1 hrs   So collective rate needed to finish the work in 1 hrs= 1/1   Let the no of machine be x   So, x/36 +x/18 =1/1   x3/36=1/1   x=12   So no of machine R is 12   Answer 12
<result>E</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 1 hours, how many machines of type R were used?