-80% is the change from 100 to 20
Calculation steps:
[( 20 - 100 ) ÷ 100] x 100 = -80%
Formula
Percentage change = [(20 - 100) / 100] x 100
Subtract the new number by the original number
20 - 100 = -80
-80 ÷ 100 = -0.8
-0.8 x 100 = -80%
Example