-6.25% is the change from 640 to 600
Calculation steps:
[( 600 - 640 ) ÷ 640] x 100 = -6.25%
Formula
Percentage change = [(600 - 640) / 640] x 100
Subtract the new number by the original number
600 - 640 = -40
-40 ÷ 640 = -0.0625
-0.0625 x 100 = -6.25%