Task: Question Answering
Input (English): Problem: a metal company ' s old machine makes bolts at a constant rate of 100 bolts per hour . the company ' s new machine makes bolts at a constant rate of 150 bolts per hour . if both machines start at the same time and continue making bolts simultaneously , how many minutes will it take the two machines to make a total of 500 bolts ? 
Options: a ) 36 , b ) 72 , c ) 120 , d ) 144 , e ) 180
Output (English): c