A tap can fill a tank in 20 hours. After half the tank is filled, three more similar taps are opened. What is the total time taken to fill the tank completely? Choose the correct choice:
A) 11 hrs 30 mins B) 10 hrs 30 mins C) 12 hrs D) 12 hrs 30 mins E) 2 hrs 30 mins
Explanation:   A tap can fill a tank in 20 hours.   Therefore the tap can fill half the tank in 10 hours.   One tap take time to fill half of the tank = 10 hrs   4 taps take time to fill half of the tank =10/4 hrs = 2 1/2 hrs   Total time taken to fill the tank = 10 hr+2 1/2 hr = 12 hrs 30 mins   Answer : 12 hrs 30 mins
<result>D</result>
A tap can fill a tank in 20 hours. After half the tank is filled, three more similar taps are opened. What is the total time taken to fill the tank completely?