-90% is the change from 420 to 42
Calculation steps:
[( 42 - 420 ) ÷ 420] x 100 = -90%
Formula
Percentage change = [(42 - 420) / 420] x 100
Subtract the new number by the original number
42 - 420 = -378
-378 ÷ 420 = -0.9
-0.9 x 100 = -90%
Example