-79% is the change from 100 to 21
Calculation steps:
[( 21 - 100 ) ÷ 100] x 100 = -79%
Formula
Percentage change = [(21 - 100) / 100] x 100
Subtract the new number by the original number
21 - 100 = -79
-79 ÷ 100 = -0.79
-0.79 x 100 = -79%
Example