Given,
Diameter of the circular pond = 40 m
So, the radius of the pond = 20/2 = 10 m = r
Thickness (width of the path) = 2 m
As the whole view of the pond looks like a hollow cylinder.
And the height will be 20 cm = 0.2 m
So,
Thickness (t) = R – r
2 = R – 20
R = 22 m
Volume of the hollow cylinder = π (R2– r2) × h
= π (222– 202) × 0.2
= 52.8 m3
Therefore, the volume of the hollow cylinder is the required amount of sand needed to spread across to a depth of 20 m.