A
chemical plant has four tanks (A, B, C and D), each containing 1000 litres of a
chemical. The chemical is being pumped from one tank to another as follows:
From A to B @ 20 litres/minute
From C to A @ 90 litres/minute
From A to D @ 10 litres/minute
From C to D @ 50 litres/minute
From B to C @ 100 litres/minute
From D to B @ 110 litres/minute
Which tank gets emptied first and how long does it take (in minutes) to get empty after pumping starts?
(1) A, 16.66
Solution:
From A to B @ 20 litres/minute
From C to A @ 90 litres/minute
From A to D @ 10 litres/minute
From C to D @ 50 litres/minute
From B to C @ 100 litres/minute
From D to B @ 110 litres/minute
Which tank gets emptied first and how long does it take (in minutes) to get empty after pumping starts?
(1) A, 16.66
(2) C, 20
(3) D, 20
(4) D, 25
Solution follows here:
The pumping speeds
of each tank are calculated as per the below table:
Pumping
speed is added for incoming flow and subtracted for outgoing flow.
Tank

Initial
speed

A>B
20 ltr/min 
C>A
90 ltr/min 
A>D
10 ltr/min 
C>D
50 ltr/min 
B>C
100 ltr/min 
D>B
110 ltr/min 
A

0

20

70

60

60

60

60

B

0

20

20

20

20

80

30

C

0

0

90

90

140

40

40

D

0

0

0

10

60

60

50

ve sign indicates pumping out and +ve sign indicates pumping
in. Hence A and B never get emptied.
As the net pumping speed of tank D is 50, it gets emptied
first in 1000/50 = 20 minutes
Answer
(3)
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