-3.53% is the change from 850 to 820
Calculation steps:
[( 820 - 850 ) ÷ 850] x 100 = -3.53%
Formula
Percentage change = [(820 - 850) / 850] x 100
Subtract the new number by the original number
820 - 850 = -30
-30 ÷ 850 = -0.03529411764705882
-0.03529411764705882 x 100 = -3.53%