-50% is the change from 200 to 100
Calculation steps:
[( 100 - 200 ) ÷ 200] x 100 = -50%
Formula
Percentage change = [(100 - 200) / 200] x 100
Subtract the new number by the original number
100 - 200 = -100
-100 ÷ 200 = -0.5
-0.5 x 100 = -50%
Example