You'll need to specify the accuracy to which you want your answer. For example: "Find the volume of this cone to 3 decimal place accuracy."
The formula for the volume of a cone is V = (1/3) (base) (height), where "base" represents the area of the base.
Here the diameter of the base is 24 feet, so the radius of the base is 12 feet.
Thus, the area of the base is (12 feet)^2 times pi: A = 144 pi ft^2.
Multiply this area by the height of the cone, which is 10 feet:
V = 1440 pi ft^3
By calculator, this volume is 4523.893421 cubic feet.
If you want this volume to the nearest cubic foot, it'd be 4524 cubic ft.
If you want this vol. to the nearest 100th cubic ft., it'd be 4523.89 cu. ft.