As per the question,

Diameter of the circular pond $=40m$

So, the radius of the pond $=40/2=20m=r$

Thickness (width of the path) $=2m$

As the whole view of the pond looks like a hollow cylinder.

And the height will be $20cm=0.2m$

So,

Thickness (t) $=R–r$

$2=R–20$

$R=22m$

Formula for volume of the hollow cylinder $=\pi \left( {{R}^{2}}-{{r}^{2}} \right)\times h$

$=\pi \left( {{22}^{2}}-{{20}^{2}} \right)\times 0.2$

$=52.8{{m}^{3}}$

Hence, the volume of the hollow cylinder is the required amount of sand needed to spread across to a depth of $20m$.