-90% is the change from 400 to 40
Calculation steps:
[( 40 - 400 ) ÷ 400] x 100 = -90%
Formula
Percentage change = [(40 - 400) / 400] x 100
Subtract the new number by the original number
40 - 400 = -360
-360 ÷ 400 = -0.9
-0.9 x 100 = -90%
Example