The answer is 72.359 grams.
To arrive to the answer, find first the percent composition of Fe₃O₄. Below is the percent composition by element of Fe₃O₄.
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The 100.0g is the mass of your sample. To get the mass of Fe₃, you multiply the mass of your sample (100.0g) by the percentage of Fe₃ (72.359%) - 100.0 g x 0.72359 = 72.359 g.
In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.
If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100.
Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. These relative weights computed from the chemical equation are sometimes called equation weights.
Using the chemical formula of the compound and the periodic table of elements, we can add up the atomic weights and calculate molecular weight of the substance. See more at ConvertUnits.com.