100% is the change from 21 to 42
Calculation steps:
[( 42 - 21 ) ÷ 21] x 100 = 100%
Formula
Percentage change = [(42 - 21) / 21] x 100
Subtract the new number by the original number
42 - 21 = 21
21 ÷ 21 = 1
1 x 100 = 100%
Example