-9.09% is the change from 99 to 90
Calculation steps:
[( 90 - 99 ) ÷ 99] x 100 = -9.09%
Formula
Percentage change = [(90 - 99) / 99] x 100
Subtract the new number by the original number
90 - 99 = -9
-9 ÷ 99 = -0.09090909090909091
-0.09090909090909091 x 100 = -9.09%