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:

A and B together fill at 1/10 + 1/40 = 1/8 tank per hour, taking 8 hours. With C open, time is 8 hours 80 minutes = 28/3 hours, so combined rate is 3/28 tank/hour. C's emptying rate = 1/8 - 3/28 = 1/56 tank/hour. After A and B fill for 7 hours, tank has 7/8 water. C empties 7/8 ÷ 1/56 = 49 hours. Option A (42) would require C's rate as 1/48, and Option B (45.5) is not consistent with the calculations.