-20.16% is the change from 640 to 511
Calculation steps:
[( 511 - 640 ) ÷ 640] x 100 = -20.16%
Formula
Percentage change = [(511 - 640) / 640] x 100
Subtract the new number by the original number
511 - 640 = -129
-129 ÷ 640 = -0.2015625
-0.2015625 x 100 = -20.16%