First, we need to convert all units to ml.
The standard conversion is the 1 liter = 1000 ml
Therefore:
3.5 liter = 3.5 * 1000 = 3500 ml
To get the difference in the amounts water between the two bottles, simply subtract the smaller amount from the larger amount as follows:
difference in amount = 3750 - 3500 = 250 ml
This means that the larger bottle has 250 ml of water more than the smaller bottle