Hi vaibhav101,
To start, you are not expected to know that a cubic cm = .001 liters. If this were an Official GMAT Question, then you would be given that 'conversion' in the prompt.
We're told that a water tank of 10,000 cm^3 capacity is to be filled from 2 taps: X and Y; the inflow of water from X is at the rate of 1 litre/min and the inflow from Y is 2.5 litres/min. We're asked for the number of MINUTES it will take for both the taps together to fill the whole tank. This question is essentially just a big 'rate' question, so you don't need to use the Work Formula to solve it.
Assuming that the tank starts off 'empty', the two taps will insert (1+2.5) = 3.5 liters/minute into the tank.
With a 10,000 cm^3 tank, you would need (10,000)(.001) = 10 liters of water to fill the tank
(10 liters)/(3.5 liters/min) = (10)/(7/2) = 20/7 minutes = 2 6/7 minutes
Final Answer: D
GMAT assassins aren't born, they're made,
Rich
