A tap can fill a tank in 6 hours. After half the tank is filled, three more similar taps are opened. What is the total time taken to fill the tank completely?
a. 3 hrs 15 min
b. 3 hrs 45 min
c. 4 hrs
d. 4 hrs 15 min
a. 3 hrs 15 min
b. 3 hrs 45 min
c. 4 hrs
d. 4 hrs 15 min