Pipes A, B and C can fill an empty tank $\frac{30}{7}$ hours, if all the three pipes are opened simultaneously. A and B are filling pipes and C is an emptying pipe. Pipe A can fill the tank in 15 hours and pipe C can empty it in 12 hours. In how long (in hours) can pipe B alone fill the empty tank?

4

A + B + C = \(\frac{30}{7}\) hrs,

A+ = 15 hrs,

C- = 12 hrs,

⇒ A + B + C = 14,   (Efficiency)

⇒ 4 + B - 5 = 14,

⇒ B - 1 = 14,

⇒ B+ = 15 units,

Time required by B to completely fill the cistern alone =  \(\frac{60}{15}\) = 4 hrs.