A quantity rises from 80 to 100. What is the percentage increase?

Percentage increase is calculated using the original value as the base. The rise is 100 − 80 = 20. Dividing 20 by the original 80 gives 0.25, and multiplying by 100 gives 25%. Using the new value of 100 as the denominator would answer a different question, so always identify the initial value.