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 20 hrs and a machine of type S does the same job in 30 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? Choose the correct choice from the following options:
A) 4 B) 5 C) 6 D) 8 E) 9
Type R completes 1/20 of the job each hour.   Type S completes 1/30 of the job each hour.   Together R and S complete 1/20 +1/30 = 1/12 of the job each hour.   Let n be the number of each type of machine.   2*n*(1/12) = 1 job completed   n = 6   The answer is 6.
<result>C</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 20 hrs and a machine of type S does the same job in 30 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?