One way to picture this is to think of a propeller. As the propeller spins, it carves out a 3D space even though the blade is "2D" in a sense.
If we spin everything around segment BC, we get a 3D cone forming. The base of the cone is vertical and it has a radius of AC. The height of the cone is segment BC. It might help to rotate the paper 90 degrees clockwise so that BC is vertical.
If each bag covers 1.25 ft^2 then you need to divide the total area 60 ft^2 by 1.25 ft^2 and that will tell you the number of bags needed which is 48 bags
