31.25% is the change from 640 to 840
Calculation steps:
[( 840 - 640 ) ÷ 640] x 100 = 31.25%
Formula
Percentage change = [(840 - 640) / 640] x 100
Subtract the new number by the original number
840 - 640 = 200
200 ÷ 640 = 0.3125
0.3125 x 100 = 31.25%