Arithmetic-18 (CAT-2005)

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

(2) C, 20

(3) D, 20

(4) D, 25

Solution follows here:

Solution:       

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)