-18.75% is the change from 640 to 520
Calculation steps:
[( 520 - 640 ) ÷ 640] x 100 = -18.75%
Formula
Percentage change = [(520 - 640) / 640] x 100
Subtract the new number by the original number
520 - 640 = -120
-120 ÷ 640 = -0.1875
-0.1875 x 100 = -18.75%