A tank can be filled by two pipes A and B in 60 minutes and 40 minutes respectively. How many minutes will it take to fill the tank from empty state if B is used for the first half time and then A and B fill it together for the other half. Choose the correct answer:
A) 15 mins
B) 20 mins
C) 25 mins
D) 30 mins
E) 35 mins
Explanation:   Let the total time be x mins.   Part filled in first half means in x/2 = 1/40   Part filled in second half means in x/2 =1/60 +1/40 =1/24   Total =X/2 X 1/40 + X/2 X 1/24 =1   x=30mins   ANSWER IS 30 mins
<result>D</result>
A tank can be filled by two pipes A and B in 60 minutes and 40 minutes respectively. How many minutes will it take to fill the tank from empty state if B is used for the first half time and then A and B fill it together for the other half.