-50.78% is the change from 640 to 315
Calculation steps:
[( 315 - 640 ) ÷ 640] x 100 = -50.78%
Formula
Percentage change = [(315 - 640) / 640] x 100
Subtract the new number by the original number
315 - 640 = -325
-325 ÷ 640 = -0.5078125
-0.5078125 x 100 = -50.78%