Question

A water from a little lake drained a total of 3.5 liters in 4 days

Answer 1

So, in 4 days, the lake lost 3.5 liters, and you need to solve for how many were lost each day. 
There are two ways to do this; one involves writing an equation, the other does not. It pretty much just depends what your teacher expects from you.

The easiest way is to simply divide 3.5 by 4; this will tell you how many liters were lost each of those four days.

The other way does the same thing, but a little more algebraically...
3.5 liters per day were lost- this can be written as a ratio: 3.5 liters/ 4 days.
You need to solve for x liters/ 1 day.
Since these are equivalent, you set them equal to each other: 
$\frac{3.5 liters}{4 days} = \frac{x liters}{1 day}$
From here, you can cross multiply to get 3.5=4x
Then simply divide both sides by 4 (to isolate the variable) and you get 1.5=x.
Since the variable x represented the number of liters lost in 1 day, you can write your answer as 1.5 liters. 

So, the simple answer is: the average change in the water volume each day was 1.5 liters.