-21.09% is the change from 811 to 640
Calculation steps:
[( 640 - 811 ) ÷ 811] x 100 = -21.09%
Formula
Percentage change = [(640 - 811) / 811] x 100
Subtract the new number by the original number
640 - 811 = -171
-171 ÷ 811 = -0.21085080147965474
-0.21085080147965474 x 100 = -21.09%