30.94% is the change from 640 to 838
Calculation steps:
[( 838 - 640 ) ÷ 640] x 100 = 30.94%
Formula
Percentage change = [(838 - 640) / 640] x 100
Subtract the new number by the original number
838 - 640 = 198
198 ÷ 640 = 0.309375
0.309375 x 100 = 30.94%