-50% is the change from 800 to 400
Calculation steps:
[( 400 - 800 ) ÷ 800] x 100 = -50%
Formula
Percentage change = [(400 - 800) / 800] x 100
Subtract the new number by the original number
400 - 800 = -400
-400 ÷ 800 = -0.5
-0.5 x 100 = -50%
Example