-21.06% is the change from 660 to 521
Calculation steps:
[( 521 - 660 ) ÷ 660] x 100 = -21.06%
Formula
Percentage change = [(521 - 660) / 660] x 100
Subtract the new number by the original number
521 - 660 = -139
-139 ÷ 660 = -0.2106060606060606
-0.2106060606060606 x 100 = -21.06%