A tap can fill a tank in 6 hours. After half the tank is filled i.e. after 3 hrs. Three more similar taps are opened i.e. no. of taps to fill remaining half tank = 4 taps when 1 tap is used = (3 hrs/1 tap) when 4 taps are used = (3 hrs/4 taps)=45min Total = 3 hrs + 45 min ∴ Total Time = 3 hours45 minutes.