Pipe A alone fills 30% of a tank in 12 hours. Pipe B alone fills the remaining part of the tank in 7 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 30% of the tank = 12 hours Or, time taken by Pipe A to fill 100% of the tank = 12 × (100 / 30) = 40 hours Remaining part of the tank = 100% - 30% = 70% Time taken by Pipe B to fill 70% of the tank = 7 hours Or, time taken by Pipe B to fill 100% of the tank = 7 × (100 / 70) = 10 hours So, total capacity of the tank = LCM(40, 10) = 40 units Efficiency of Pipe A = 40 / 40 = 1 unit/hour Efficiency of Pipe B = 40 / 10 = 4 units/hour Combined efficiency of Pipe A and Pipe B = 1 + 4 = 5 units/hour So, time taken by both pipes together to fill the tank = 40 / 5 = 8 hours Hence, the correct answer is B.