Please solve with steps

a) Consider a cave to be well approximated as a perfectly spherical, air-filled cavity within the subsurface. Let the centre of the cave be 50 m below the ground surface and the background geology to consist of limestone of density 2600 kg/m?*. If you can measure gravity to an accuracy of 0.1 mGal, what must the radius of the cave be in order that it is detectable by your survey? b) Now suppose that the cave is of radius 20 m, and that is there is box of dense material at the very bottom. By modelling the box to be a sphere of radius 1.5 m, what must the density of the box be in order that there is zero net gravitational anomaly on the surface directly above the cave? Would this mean that the box and cave are undetectable