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	Choices:
A) 15 mins  B) 20 mins  C) 25 mins  D) 30 mins  E) None of these
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∗1/40+x/2∗1/24=1   =&gt;x/2(1/40+1/24)=1   =&gt;x/2∗1/15=1   =&gt;x=30mins   Option 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