-9.9% is the change from 101 to 91
Calculation steps:
[( 91 - 101 ) ÷ 101] x 100 = -9.9%
Formula
Percentage change = [(91 - 101) / 101] x 100
Subtract the new number by the original number
91 - 101 = -10
-10 ÷ 101 = -0.09900990099009901
-0.09900990099009901 x 100 = -9.9%