Two pipes A and B can fill in 40 minutes and 30 minutes respectively.Both the pipes are open together but after 5 minutes, pipe B is turned off.What is the total time required to fill the tank?

Answer 1

we see that both pipes together can fill the the tank in 120/7 minutes:

1/t = 1/40 + 1/30
t = 7/120

So, after 5 minutes, they have filled 35/120 = 7/24 of the tank.

So, it will take A 7/24*40 = 35/3 minutes to fill the rest.

Total time: 5 + 35/3 = 16 2/3 minutes.

Answer 2

T = Ta*Tb/(Ta+Tb) = 40*30/(40+30)= 17.14 Min. to fill with both pipes open.

(17.14-5)/17.14 = 0.708 of a tank
to be filled.

5 + 0.708*40 = 33.34 Min., total.