If the length of the cylinder is c and the radius of the cone and cylinder is r, and the height of the cone is h, the volume of the cylinder is πr^2c (which is also the volume of sand), and the volume of the cone is πr^2h/3. The combined volume is πr^2(c+h/3).
If the sand is used to fill the cone first the surplus of sand will be πr^2c-πr^2h/3=πr^2(c-h/3).
This surplus is used to part fill the cylinder to a depth of x, so that πr^2x=πr^2(c-h/3) and x=c-h/3.
There is no more information or picture to put values for c or h, but if c=h x=2h/3 or 2c/3, that is, two thirds the height of the cone or cylinder.