Ghulam,
The essential concept here is similar triangles.
In the diagram above I have let r be the radius of the circle formed by the water surface. The triangles PQR and PTS are similar and hence
^{QR}/_{RP} = ^{TS}/_{SP} that is ^{8}/_{20} = ^{r}/_{x}
Hence
r = ^{2}/_{5} x
The volume of a cone of radius r and height h is
V = ^{1}/_{3} r^{2} h
and hence the volume of water in your cone is
V = ^{1}/_{3} r^{2} x = ^{1}/_{3} ( ^{2}/_{5} x)^{2} x = ^{4}/_{75} x^{3}
Penny
