100% is the change from 210 to 420
Calculation steps:
[( 420 - 210 ) ÷ 210] x 100 = 100%
Formula
Percentage change = [(420 - 210) / 210] x 100
Subtract the new number by the original number
420 - 210 = 210
210 ÷ 210 = 1
1 x 100 = 100%
Example