Question

How many atoms are in 15.3 cm3 of mercury (at room temperature)?

Answer 1

you need to know the radius of a single Hg atom, and the volume of a sphere

then find the volume of of a single Hg atom,
and find how many times the volume of one atom fits into 15.3 cm^3

Answer 2

well, you have the density of mercury from tables. About 13.6 g/ml. You have the molar mass of mercury in g/mol. So convert volume 15.3 ml to g using density.
Convert g to moles using molar mass, then convert moles to number using Avogadros' number