-4.2% is the change from 666 to 638
Calculation steps:
[( 638 - 666 ) ÷ 666] x 100 = -4.2%
Formula
Percentage change = [(638 - 666) / 666] x 100
Subtract the new number by the original number
638 - 666 = -28
-28 ÷ 666 = -0.042042042042042045
-0.042042042042042045 x 100 = -4.2%