The first tap fills the tank at a rate of 61 tanks per hour. After half the tank is filled, it takes 3 hours to fill that half.
Now, with four taps (the original plus three more), the rate becomes 4×61=32 tanks per hour.
To fill the remaining half tank, the time required is 2/31/2=21×23=43 hours.
Thus, the total time taken to fill the tank completely is 3+43=3.75 hours.