Most importantly let us make sure that you understand the problem.

I have attached an illustration to help you understan the question.

As you see, I have drawn three tracks; 1 is the inside track and 3 is the outside track. The straight aways for all three are equal length. Here I am talking about the horizontal black lines that are between the vertical red lines.

However the semicircles on the end are all of different radius. The perimeter of a semicircle is so the distances for the three lanes are different.

Because the length of each track is different but we want the finish line for all runners to be at the same point we must have those on the inside lane start further back than the rest of the runners.

Make sense?