XAT 2017 Question 47
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XAT
Let us assume the volume of the tank to be 60 litres.
A can fill or empty 60/4 = 15 litres in a minute.
B can fill or empty 60/10 = 6 litres in a minute.
C can fill or empty 60/12 = 5 litres in a minute.
D can fill or empty 60/20 = 3 litres in a minute.
We have to find the combination for which the tank will be half-full in 30 minutes (i.e., completely filled in 1 hour).
Therefore, the combination must result in a net input of 60/60 = 1 litre per minute.
Let us evaluate the options.
Option B:
Pipe A drained and pipes B, C and D filled
The net result will be -15 + 6 + 5 + 3 = -1 litre/minute. We can eliminate option B.
Option C:
Pipes A and D drained and pipes B and C filled
The net result will be -15-3+6+5 = -7 litres/minute. We can eliminate option C as well.
Option D:
Pipes A and D filled and pipes B and C drained
The net result will be 15+3-6-5 = 7 litres/minute. We can eliminate option D as well.
Option A:
Pipe A filled and pipes B, C and D drained
The net result will be 15-6-5-3 = 1 litre/minute.
Therefore, option A is the right answer.