-20.56% is the change from 321 to 255
Calculation steps:
[( 255 - 321 ) ÷ 321] x 100 = -20.56%
Formula
Percentage change = [(255 - 321) / 321] x 100
Subtract the new number by the original number
255 - 321 = -66
-66 ÷ 321 = -0.205607476635514
-0.205607476635514 x 100 = -20.56%