-50% is the change from 680 to 340
Calculation steps:
[( 340 - 680 ) ÷ 680] x 100 = -50%
Formula
Percentage change = [(340 - 680) / 680] x 100
Subtract the new number by the original number
340 - 680 = -340
-340 ÷ 680 = -0.5
-0.5 x 100 = -50%
Example