Task: Question Answering
Input (English): Problem: 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 9 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 ? 
Options: a ) 3 , b ) 4 , c ) 18 / 5 , d ) 9 , e ) 12
Output (English): c