-21.6% is the change from 639 to 501
Calculation steps:
[( 501 - 639 ) ÷ 639] x 100 = -21.6%
Formula
Percentage change = [(501 - 639) / 639] x 100
Subtract the new number by the original number
501 - 639 = -138
-138 ÷ 639 = -0.215962441314554
-0.215962441314554 x 100 = -21.6%