Pipe A can fill an empty cistern alone in 26 hours and pipe B can fill the same cistern alone in 36 hours. The time taken by them to fill half of the cistern by operating together will be:
15 \( \frac{17}{31} \) hours
11 \( \frac{17}{31} \) hours
7 \( \frac{17}{31} \) hours
16 \( \frac{17}{31} \) hours