Tap A can fill a tank in 6 hours, tap B can fill the same tank in 8 hours and tap C can empty the same tank in 4 hours. If all three taps A, B and C are opened together, then how much time (in hours) will be taken to fill the tank?

24 30 28 20

24

A+ = 6 hrs, B+ = 8 hrs, C- = 4 hrs, ⇒ Time required by A + B + C to fill the tank completely = \(\frac{24}{4+3+-6}\) = \(\frac{24}{1}\) = 24 hrs.