-0.31% is the change from 641 to 639
Calculation steps:
[( 639 - 641 ) ÷ 641] x 100 = -0.31%
Formula
Percentage change = [(639 - 641) / 641] x 100
Subtract the new number by the original number
639 - 641 = -2
-2 ÷ 641 = -0.0031201248049922
-0.0031201248049922 x 100 = -0.31%