Pipes A and B can fill a tank in 10 hours and 40 hours, respectively. C is an outlet pipe attached to the tank. If all the three pipes are opened simultaneously, it takes 80 minutes more time than what A and B together take to fill the tank. A and B are kept opened for 7 hours and then closed and C was opened. C will now empty the tank in:

49 hours

A+ = 10 hrs , B+ = 40 hrs,

⇒ Time required for A + B to fill the tank completely = \(\frac{40}{5}\) = 8 hrs.

⇒ C opened with A + B, it takes 9 hours and 20 minutes to fill the tank,

⇒ A + B opened for 7 hours = (28 + 7) x 7 = 35 x 7 = 245 units,

⇒ A + B + C = 30,  (Efficiency)

⇒ 35 + C = 30,

⇒ C = -5,

⇒ Time required for C to empty 245 units of tank = \(\frac{245}{5}\) = 49 hours.