How many argon (ar) atoms are there in 1.5 × 102 g of argon?

Answer is: there is 2,25·10²⁴ argon atoms.
m(Ar) = 1,5·10² g = 150 g.
n(Ar) = m(Ar) ÷ M(Ar).
n(Ar) = 150 g ÷ 40 g/mol.
n(Ar) = 3,75 mol.
N(Ar) = n(Ar) · Na.
N(Ar) = 3,75 mol · 6·10²³ 1/mol.
N(Ar) = 2,25·10²⁴.
n - amount of substance.
Na - Avogadro number.
M - molar mass of atom.
m - mass of atom.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.