-20% is the change from 850 to 680
Calculation steps:
[( 680 - 850 ) ÷ 850] x 100 = -20%
Formula
Percentage change = [(680 - 850) / 850] x 100
Subtract the new number by the original number
680 - 850 = -170
-170 ÷ 850 = -0.2
-0.2 x 100 = -20%
Example