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 4 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?  Choose the correct choice from the following options: A) 3 B) 4 C) 9 / 5 D) 9 / 8 E) 12

Rationale: "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 4 hours . if the company used the same number of each type of machine to do job in 2 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 / 4 lets say x machines are used to attain the desired rate , thus x / 36 + x / 4 = 1 / 2 ( desired r = 1 / 2 i . e . to complete the job in 2 hours ) ( x + 9 x ) / 36 = 1 / 2 10 x / 36 = 1 / 2 x = 9 / 5 . qa = 9 / 5 ( answer c )"
Answer: C