-21.6% is the change from 815 to 639
Calculation steps:
[( 639 - 815 ) ÷ 815] x 100 = -21.6%
Formula
Percentage change = [(639 - 815) / 815] x 100
Subtract the new number by the original number
639 - 815 = -176
-176 ÷ 815 = -0.21595092024539878
-0.21595092024539878 x 100 = -21.6%