Aswimming pool had 2 . 5 2.52, point, 5 million liters of water in it. some water evaporated, and then the pool only had 2 22 million liters of water in it.

The correct question is
A swimming pool has 2.5 million liters of water, some water evaporated and now there is only 2 million liters. How much percent evaporated?

Let
x > initial swimming pool  liters of water 
y > final swimming pool  liters of water
z > percent evaporated liters of water

z=[(x-y)/x]*100

we know that
x=2.5 million liters
y= 2 million liters
therefore
z=[(2.5-2)/2.5]*100=20 %

the answer is 20%