-50% is the change from 170 to 85
Calculation steps:
[( 85 - 170 ) ÷ 170] x 100 = -50%
Formula
Percentage change = [(85 - 170) / 170] x 100
Subtract the new number by the original number
85 - 170 = -85
-85 ÷ 170 = -0.5
-0.5 x 100 = -50%
Example