inner and outer edges constitute two concentric circles.
If the width of the border is , the the area of the border is:
That is it is the difference of area between the circle of radius and one of radius .
Then the volume of cement required is:
so. as we have:
rearranging this gives:
This is a quadratic in which can be solved using the quadratic formula to give:
The second of these is negative and so cannot be the solution which leaves the first, so: