To figure out the size of a rectangle that would represent the swimming pool, you will use the formula for volume of a rectangle or prism to solve for the area of the base.
V = Bh
1200 = 5B
B = 240 square feet
To get an area of 240 ft.², you could use the dimensions 10' x 24'. Another possibility would be 8' x 30'.
I thnk 83 if the year has 365 days
The volume of the shape
We have a cylinder that is cut into two. Its external radius is 5ft, while its internal radius is 4ft.
External radius = internal radius + thickness

Internal radius = 4ft (given)
Recall that the volume(V) of a full cylinder is:

Since the cylinder is cut into two, the volume(V) is:

height (h) = 12ft (given)
The volume of the shape: