100% is the change from 400 to 800
Calculation steps:
[( 800 - 400 ) ÷ 400] x 100 = 100%
Formula
Percentage change = [(800 - 400) / 400] x 100
Subtract the new number by the original number
800 - 400 = 400
400 ÷ 400 = 1
1 x 100 = 100%
Example