Pipe A alone fills 75% of a tank in 18 hours. Pipe B alone fills the remaining part of the tank in 3 hours. In how many hours will the tank be completely filled if both pipes are opened together?

ATQ, Time taken by Pipe A to fill 75% of the tank = 18 hours Or, time taken by Pipe A to fill 100% of the tank = 18 × (100 / 75) = 24 hours Remaining part of the tank = 100% - 75% = 25% Time taken by Pipe B to fill 25% of the tank = 3 hours Or, time taken by Pipe B to fill 100% of the tank = 3 × (100 / 25) = 12 hours So, total capacity of the tank = LCM(24, 12) = 24 units Efficiency of Pipe A = 24 / 24 = 1 unit/hour Efficiency of Pipe B = 24 / 12 = 2 units/hour Combined efficiency of Pipe A and Pipe B = 1 + 2 = 3 units/hour So, time taken by both pipes together to fill the tank = 24 / 3 = 8 hours Hence, the correct answer is B.