-90% is the change from 800 to 80
Calculation steps:
[( 80 - 800 ) ÷ 800] x 100 = -90%
Formula
Percentage change = [(80 - 800) / 800] x 100
Subtract the new number by the original number
80 - 800 = -720
-720 ÷ 800 = -0.9
-0.9 x 100 = -90%
Example