-20.13% is the change from 800 to 639
Calculation steps:
[( 639 - 800 ) ÷ 800] x 100 = -20.13%
Formula
Percentage change = [(639 - 800) / 800] x 100
Subtract the new number by the original number
639 - 800 = -161
-161 ÷ 800 = -0.20125
-0.20125 x 100 = -20.13%