-5% is the change from 420 to 399
Calculation steps:
[( 399 - 420 ) ÷ 420] x 100 = -5%
Formula
Percentage change = [(399 - 420) / 420] x 100
Subtract the new number by the original number
399 - 420 = -21
-21 ÷ 420 = -0.05
-0.05 x 100 = -5%
Example