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.