A water tank is 2/5th full pipe A can fill the tank in 10 minutes and the pipe B can empty it in 6 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely?

Let Y be closed after T min
Then, T x (1/ 24 + 1/32) + (18 - T) x 1/24 = 1
? (7 x T)/96 + (18 - T)/24 = 1
or 7T + 72 - 4T = 96.
? 3T = 24
or T = 8 min.

Part filled by inlet in 1 hour = (1/6 - 1/8) = 1/24
So, the inlet can fill the tank in 24 hours
? Capacity of the tank = Water that flows in 24 hours = (4 x 24 x 60) liters = 5760 liters

Part filled in 5 min. = 5 x (1/12 + 1/15) = 5 x 9/60 = 3/4

Part emptied in 1 min. (when all the pipes are opened) = 1/6 - (1/12 + 1/15) = (1/6 - 3/20) = 1/60

Now, 1/60 part is emptied in 1 min. 3/4 part will be emptied in (60 x 3/4) = 45 min.

[(A's 1 hour work) + (A + B)'s 1 hour work]
= (1/10) + [(1/10) + (1/12)] = 17/60
Remaining part = 1 - (17/60) = 43/60
Now, (A + B + C)'s 1 hour work = (1/10) + (1/12) + (1/15) = 1/4

1/4 part is filled by 3 pipes in 1 hour.
43/60 part will be filled by them in 4 x (43/60) hrs. = 2 hours 52 min.

? Total time taken to fill the cistern = 4 hours 52 min.

12(1 - t/25) = 3
? t = 45/4 = 11¹/₄ minutes
? Required answer = 11¹/₄ - 3
= 8¹/₄ minutes

Required answer = (25 x 40 x 30) / (40 x 30 + 25 x 30 - 25 x 40)
= 600 / 19 = 31¹¹/₁₉ minutes

Time taken to fill the whole tank = (12 x 14 x 8) / (14 x 8 + 12 x 8 - 12 x 14) = 168/5 minutes
? In 7 minutes 5/168 x 7 = 5/24 part of the tank will be filled
? Requied answer = 1 - 5/24 = 19/24 part

Let the first pipe be shut up after x minutes
Now, applying the given rule, we have 30{1 - (x + 10)/40 } = x
[Here t = (x + 10) minutes]
or x = 90/7 minutes

Here w = 2 litres per hour
? Required answer = {(15 x 10) / (15 - 10)} x 2 = 60 litres.

Part filled by tap A in 1 min = 1/60
Let tap B fills the tank in x min
Then, Part filled by tap, B in 1 min = 1/x
According to the question,
1/60 + 1/x = 1/40
? 1/x = 1/40 - 1/60
&rArr ; 1/x = (3 - 2)/120
&rArr ; 1/x = 1/120
? Tap B can fill the tank in 120 min.