-2.5% is the change from 640 to 624
Calculation steps:
[( 624 - 640 ) ÷ 640] x 100 = -2.5%
Formula
Percentage change = [(624 - 640) / 640] x 100
Subtract the new number by the original number
624 - 640 = -16
-16 ÷ 640 = -0.025
-0.025 x 100 = -2.5%
Example