40% is the change from 15 to 21
Calculation steps:
[( 21 - 15 ) ÷ 15] x 100 = 40%
Formula
Percentage change = [(21 - 15) / 15] x 100
Subtract the new number by the original number
21 - 15 = 6
6 ÷ 15 = 0.4
0.4 x 100 = 40%
Example