Two pipes A and B can fill a tank in 12 hours and 18 hours, respectively. Both pipes are opened simultaneously. In how much time will the empty tank be filled completely?

7 hours 12 minutes

A+ = 12 hrs, B+ = 18 hrs,

⇒ Time required for A + B to fill the cistern completely = \(\frac{36}{3+2}\) = \(\frac{36}{5}\) = \( { 7}_{ 5}^{ 1} \) hrs = 7 hours 12 minutes.