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 4 hours, how many machines of type R were used?
Select the correct option: A) 3 B) 4 C) 6 D) 9 E) 12
Rate of machine R = 1/36   Rate of machine S = 1/18   Rate of R+S = 1/12   To finish the work in 4 hrs, the combined rate needs to be 1/4.   x * 1/12 = 1/4   x = 3   The answer is 3.
<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 4 hours, how many machines of type R were used?