Task: Question Answering
Input (English): Problem: a man can do a job in 15 days . his father takes 20 days and his son finishes it in 15 days . how long will they take to complete the job if they all work together ? 
Options: a ) 6.3 , b ) 6.9 , c ) 5.5 , d ) 6.1 , e ) 6.2
Output (English): c