Answer :

First, there are 1,000 milliliters in a liter
Now add 600+400=1000mL
So since 1000mL is one liter 
Cathy drank 1 liter
Cathy drank 1 liter 600 + 400 milliliters = 1,000 milliliters and 1,000 milliliters = 1 liter