Question 1011462
The wording of this question is not clear, so I will use my interpretation.  It matters when the water is added. I will assume the water was added at the end of the day.
x=start
x/2 left after drinking.
x/2 +200 after water added.
x/4 +100 after drinking half
200 added
Has x/4+300=500
x/4=200
x=800 ml  ANSWER
drank 400 and 200 was added
Thursday starts with 600
drank 300 and added 200.  That is 500.