P can fill a tank in 5 hours. Q can fill the same tank in 10 hours. R. can empty the same tank in 20 hours. How much time will all the three take together to fill the same tank?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 4 hours.
Total capacity of tank = 20 (LCM of 5, 10, and 20)
Filling rate of P, Q, and R = (20/5) + (20/10) - (20/20) = 4 + 2 - 1 = 5 units/hr
Total time required to fill = 20/5 = 4 hours
Hence, all the three pipes together take 4 hours to fill the same tank.