-19.06% is the change from 640 to 518
Calculation steps:
[( 518 - 640 ) ÷ 640] x 100 = -19.06%
Formula
Percentage change = [(518 - 640) / 640] x 100
Subtract the new number by the original number
518 - 640 = -122
-122 ÷ 640 = -0.190625
-0.190625 x 100 = -19.06%