How many atoms of copper are in an old penny made of pure copper and weighing 2.15 g?

Molar mass of copper = 63.55 g/mol
No. of moles of copper in the old penny = (2.15 g) / (63.55 g/mol) = 2.15/63.55 mol

1 moles of copper contains 6.022 × 10²³ copper atoms.
No. of copper atoms = (6.022 × 10²³ atoms/mol) × (2.15/63.55 mol) = 2.04 × 10²² atoms

Moles(Cu) = 2.15g / 63.5 (Ar)
mol(Cu) = 0.0338..

One mole of a substance contains 6.022 x 10^23 atoms (The Avogadro Number).

Hence
atoms (Cu) = 0.0338 x 6.022 x 10^23 = 2.03 x 10^22 atoms of Cu.

Clue - use Avogadro's number and the atomic weight of copper

(2.15 / 63.5) x 6.02 x 10^23 = calculator

Be aware they weren't pure copper even before 1982, they were brass even then, now they're just copper plated zinc, but before that they were a copper zinc alloy.

2.03 x 10^22

i think