An increase from 16 to 20 is what percent of increase?
Your measure start at 16. So that is the basis (unless stated otherwise) upon which you reference the change.
For this to be a percentage we need the denominator such that it becomes 100. Thus we have:
To express this in the standard script form we have 25%
Sometimes the use of % and its meaning is misunderstood.
The % that follows the 25 really means that we have 25 parts out of 100. In other words:
To coin a phrase; 25% and