The mass of a single uranium atom is 4.70E-22 grams. How many uranium atoms would there be in 164 milligrams of uranium?
4.70E-22g/atom x # atoms = ?
How do you find # atoms? You know you have mol U = 0.164g/atomic mass = ?
Then you know there are 6.02E23 atoms in 1 mol. So
# mols x 6.02E23 atoms/mol = # atoms.