Need help in calculation!!!!

Change in the parameter ( Blood flow in this case ) divided by the original value of the parameter.
Which is the blood flow at rest condition in this case. Then you multiply it by 100.
Just explain it as the change in blood flow = blood flow during exercise - blood flow during rest which is equal to 12500 - 1000 = 11500. Which is the extra blood flow during strenuous exercise. Further the percentage change is calculated by Change in blood flow divided by the blood flow at rest which is equal to 11500/1000*100