Answer

How many atoms of lead are found in a 2 mg sample?

Reason it through carefully:

The atomic mass of lead is 210

therefore 210g lead contains N_A atoms of lead

1 g lead contains N_A/210 atoms

2 mg lead contains (N_A/210) x 2 x 10^-3 atoms

= 5.7 x 10¹⁸ atoms