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 ?  Choose the correct answer
A) 3 hrs 15 min B) 3 hrs 45 min C) 4 hrs D) 4 hrs 15 min E) None
Solution   Time taken by one tap to fill half the tank = 3hrs.   Part filled by the four taps in 1 hour	= (4×1/6)   = 2/3.   Remaining part	= (1 - 1/2)   = 1/2.   Therefore,	‹=› 2/3 : 1/2 :: 1 : x   ‹=› (1/2×1×3/2)   ‹=› 3/ 4 hrs i.e 45 mins.   So, total time taken = 3 hrs 45 min.   Answer 3 hrs 45 min
<result>B</result>
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 ?