Answer
Please see the work below.
Work Step by Step
We know that
$\frac{1 \space Fe\space atom}{1 \space Fe(NO_3)_2 \space unit}\times \frac{1 \space Fe(NO_3)_2\space unit}{2NO_3^- ions}\times \frac{1 NO_3^- ion}{3 O atoms}=\frac{1\space Fe \space atom}{6\space O atoms}=\frac{1}{6}$
