A single tap can fill a tank in 6 hours. Once the tank is half full, three additional identical taps are opened. How long will it take to fill the tank entirely?

Explanation

The first tap fills half the tank in 3 hours (since it fills the whole tank in 6 hours). After that, four taps work together to fill the remaining half. The combined rate of four taps is four times that of one tap, so they fill the remaining half in 1.5 hours. Total time = 3 hours + 1.5 hours = 4.5 hours or 3 hours 45 minutes.