An inlet pipe can fill an empty tank in 2 hours and an outlet pipe can empty the full tank in 3 hours. In what time will the empty tank get filled, if the inlet and outlet piped are opened together?

2 h $\frac{3}{2}$h 3 h 6 h

6 h

A+ = 2 hrs, B- = 3 hrs, ⇒ Time required by A + B to completely fill the tank = $\frac{6}{3-2}$ = $\frac{6}{1}$ = 6 hrs.