Task: Question Answering
Input (English): Problem: machine a and machine b process the same work at different rates . machine c processes work as fast as machines a and b combined . machine d processes work 3 times as fast as machine c ; machine d ’ s work rate is also exactly 4 times machine b ’ s rate . assume all 4 machines work at fixed unchanging rates . if machine a works alone on a job , it takes 6 hours and 20 minutes . if all 4 machines work together on the same job simultaneously , how many minutes will it take all of them to complete it ? 
Options: a ) 15 , b ) 17 , c ) 19 , d ) 21 , e ) 24
Output (English): c