-50% is the change from 502 to 251
Calculation steps:
[( 251 - 502 ) ÷ 502] x 100 = -50%
Formula
Percentage change = [(251 - 502) / 502] x 100
Subtract the new number by the original number
251 - 502 = -251
-251 ÷ 502 = -0.5
-0.5 x 100 = -50%
Example