How many moles are 2.85 times 10^18 atoms of iron?

1 mol contains $\approx 6.02 \times {10}^{23}$ particles, whether it be atoms, ions, molecules or whatever (Avogadro's number).
$\frac{2.85 \times {10}^{18}}{6.02 \times {10}^{23}} = 0.473 \times {10}^{-} 5 = 4.73 \times {10}^{-} 6$