100% is the change from 20 to 40
Calculation steps:
[( 40 - 20 ) ÷ 20] x 100 = 100%
Formula
Percentage change = [(40 - 20) / 20] x 100
Subtract the new number by the original number
40 - 20 = 20
20 ÷ 20 = 1
1 x 100 = 100%
Example