For this case we have the following data:
Drained 3.5 Liters in 4 days. If we want to know the volume of water per day, we make a rule of three:
3.5 Liters -----> 4days
x Liters --------> 1 day
Where x represents the amount of water drained each day.
So, we have to:
Liters per day
So, the average change in water volume each day is: Liters per day
Answer:
Liters per day