He had 2 1/2 liters of water. He used 5/2 liters of water to make lemonade.
To convert 5/2 to a mixed number, see how many times 2 goes into 5. It goes twice, so 2 is the whole number. The remainder after dividing 5 by 2 is 1, so this is the numerator of the fraction; this gives us 2 1/2.
This means he started with 2 1/2 liters of water and used 2 1/2 liters of it. This leaves him 0 liters of water.
