-10% is the change from 90 to 81
Calculation steps:
[( 81 - 90 ) ÷ 90] x 100 = -10%
Formula
Percentage change = [(81 - 90) / 90] x 100
Subtract the new number by the original number
81 - 90 = -9
-9 ÷ 90 = -0.1
-0.1 x 100 = -10%
Example