Task: Question Answering
Input (English): Problem: a and b together can do a piece of work in 6 days and a alone can do it in 13 days. in how many days can b alone can do it ?
Options: a. 12 days, b. 15 days, c. 78 / 7 days, d. 21 days, e. 22 / 7 days
Output (English): c