Tap A can fill an empty tank in 8 hours if no other tap is open, and tap B can empty the same tank, when filled, in 12 hours, when no other tap is open. Find the time taken to fill the same tank, when empty, if both the taps are opened together.

12 hours 16 hours 24 hours  36 hours

24 hours

A+ = 8 hrs, B- = 12 hrs, ⇒ Time required by A + (-B) to fill the tank = \(\frac{24}{1}\) = 24 hrs.