-2.5% is the change from 80 to 78
Calculation steps:
[( 78 - 80 ) ÷ 80] x 100 = -2.5%
Formula
Percentage change = [(78 - 80) / 80] x 100
Subtract the new number by the original number
78 - 80 = -2
-2 ÷ 80 = -0.025
-0.025 x 100 = -2.5%
Example