Task: Question Answering
Input (English): Problem: a can do a job in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is ?
Options: a. 2 / 15, b. 8 / 15, c. 3 / 11, d. 1 / 12, e. 6 / 13
Output (English): b