Two pipes A and B can fill a tank in 15 hours and 18 hours, respectively. Both pipes are opened simultaneously to fill the tank. In how many hours will the empty tank be filled?

$8\frac{2}{11}$

A+ = 15 hrs, B+ = 18 hrs,

⇒ Time required by A + B to fill the tank completely = \(\frac{90}{6+5}\) = \(\frac{90}{11}\) = \( {8 }_{11 }^{2 } \) hrs.