-10.99% is the change from 91 to 81
Calculation steps:
[( 81 - 91 ) ÷ 91] x 100 = -10.99%
Formula
Percentage change = [(81 - 91) / 91] x 100
Subtract the new number by the original number
81 - 91 = -10
-10 ÷ 91 = -0.10989010989010989
-0.10989010989010989 x 100 = -10.99%