Task: Question Answering
Input (English): Problem: a man can do a job in 5 days. his father takes 20 days and his son finishes it in 25 days. how long will they take to complete the job if they all work together ?
Options: a. 6.3, b. 6.9, c. 3.5, d. 6.1, e. 6.2
Output (English): c