Task: Question Answering
Input (English): Problem: two pipes a and b can fill a tank in 10 hours and 20 hours respectively. if both the pipes are opened simultaneously, how much time will be taken to fill the tank ?
Options: a. 10 / 3, b. 20 / 3, c. 15 / 7, d. 11 / 17, e. 1 / 4
Output (English): b