-19.75% is the change from 800 to 642
Calculation steps:
[( 642 - 800 ) ÷ 800] x 100 = -19.75%
Formula
Percentage change = [(642 - 800) / 800] x 100
Subtract the new number by the original number
642 - 800 = -158
-158 ÷ 800 = -0.1975
-0.1975 x 100 = -19.75%