-50% is the change from 642 to 321
Calculation steps:
[( 321 - 642 ) ÷ 642] x 100 = -50%
Formula
Percentage change = [(321 - 642) / 642] x 100
Subtract the new number by the original number
321 - 642 = -321
-321 ÷ 642 = -0.5
-0.5 x 100 = -50%
Example