To solve this problem let's first determine how long it would take an object in free fall to travel 20 m. Next, we'll figure out how far the object will fall in 0.1 seconds less time.
Time to fall from rest can be determined using this equation:
Let's take the acceleration of gravity to be
And the distance is:
So the time to fall is:
Now lets find out how far something falls in
So we can see that the pelican will cover the last 1.94 meters in just 0.1 seconds. If the fish is able to spot the pelican and react while it is still 1.94 meters above the surface, it just might escape.