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 60 hrs and a machine of type S does the same job in 90 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? Select the correct option
A) 12  B) 15  C) 16  D) 18  E) 20
Type R completes 1/60 of the job each hour.   Type S completes 1/90 of the job each hour.   Together R and S complete 1/60 +1/90 = 1/36 of the job each hour.   Let n be the number of each type of machine.   2*n*(1/36) = 1 job completed   n = 18   The answer is 18.
<result>D</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 60 hrs and a machine of type S does the same job in 90 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?