If we divide the number of Fe atoms (150x10^12) by Avogadro's Number (6.022x10^23) we get the number of moles of Iron. This value comes out to be 2.49x10^-10. If we multiply the number of moles by the molar mass of iron (55.86 grams) than we get a value that comes out to be 1.39x10^-8 grams or 13.9 nanograms.
