A company has two types of machines, type R and type S. Operating at a constant rate a machine of R does a certain job in 36 hours and a machine of type S does the job in 12 hours. If the company used the same number of each type of machine to do job in 12 hours,how many machine R were used?	Choices
A) 3 B) 4 C) 6 D) 9/2 E) 12
yes there is a typo in the question, I got the same ques on my GMAT PREP last week, and the questions goes as:   A company has two types of machines,type R and type S.Operating at a constant rate a machine of R does a certain job in 36 hours and a machine of type S does the job in 12 hours .If the company used the same number of each type of machine to do job in2 hours,how many machine R were used?   So for a job to be done in 2 hours R = 1/2   R_a(Rate of Machine R) = 1/36   R_s(Rate of Machine S) = 1/12   Lets say x machines are used to attain the desired rate, thus   x/36+x/12 = 1/2 (Desired R =1/2 i.e. to complete the job in 2 hours)   (x+3x)/36 = 1/2   4x/36 = 1/2   x = 9/2.   QA = 9/2 (ANSWER 9/2)
<result>D</result>
A company has two types of machines, type R and type S. Operating at a constant rate a machine of R does a certain job in 36 hours and a machine of type S does the job in 12 hours. If the company used the same number of each type of machine to do job in 12 hours,how many machine R were used?