This can be done by deducting the area of the green circle from the area of the outer circle.
The areas are found by using "pi r squared" where r is the radius.
Therefore if the diameter is 250 meters then the radius is 125 meters.
The outer circle is 2.5 meters wider i.e. 127.5 meters
Thus the formula is =pi()127.5^2 - pi()125^2 (the ^ raises the power of a number)
Now we know the area, we simply divide by 4 to find the number of 4 square meters. This gives the number of seconds.
Then divide that answer by 60 to give the minutes
The formula can be seen in the downloaded file at cell A36
If you haven't already downloaded the file to work on it, go to Downloads>Excel in the menu above and look for the file "mowing problem"