-20% is the change from 830 to 664
Calculation steps:
[( 664 - 830 ) ÷ 830] x 100 = -20%
Formula
Percentage change = [(664 - 830) / 830] x 100
Subtract the new number by the original number
664 - 830 = -166
-166 ÷ 830 = -0.2
-0.2 x 100 = -20%
Example