A tap can fill a tank in 8 hrs. After half the tank is filled, three more similar taps are opened. What is the total time taken to fill the tank completely?

Time taken by 1 tap to fill the whole tank $=8$ hrs.Time taken by 1 tap to fill half the tank $=4$ hrs.Now, the remaining half tank needs to be filled.Number of taps working now $=1(\text{original})+3(\text{new})=4$ taps.Efficiency of 4 taps $=4\times$ Efficiency of 1 tap.Time for 4 taps to fill full tank $=\frac{8}{4}=2$ hrs.Time for 4 taps to fill half tank $=\frac{2}{2}=1$ hr.Total time $=4$ hrs (1st half) $+1$ hr (2nd half) $=5$ hrs.