-22.07% is the change from 820 to 639
Calculation steps:
[( 639 - 820 ) ÷ 820] x 100 = -22.07%
Formula
Percentage change = [(639 - 820) / 820] x 100
Subtract the new number by the original number
639 - 820 = -181
-181 ÷ 820 = -0.22073170731707317
-0.22073170731707317 x 100 = -22.07%