-50% is the change from 640 to 320
Calculation steps:
[( 320 - 640 ) ÷ 640] x 100 = -50%
Formula
Percentage change = [(320 - 640) / 640] x 100
Subtract the new number by the original number
320 - 640 = -320
-320 ÷ 640 = -0.5
-0.5 x 100 = -50%
Example