Question

Pipe A can fill a tank in 3 hrs. Pipe B can fill the tank in 4 hrs. Pipe C can empty a full tank in 6 hrs. If all 3 pipes are kept open simultaneously, how long will it take to fill the tank half?

Answer 1

time taken=
$\frac{1}{3} + \frac{1}{4} - \frac{1}{6} = \frac{4 + 3 - 2}{12} \\ = \frac{5}{12}$
so time = reciprocal of 5/12=12/5=2.4 hrs