Question

If you wish to produce 100g of Iron Oxide, how many grams of Iron will you need?

Answer 1

You can do this two ways. The long way is to use stoichiometry. Here is a worked example.
http://www.jiskha.com/science/chemistry/stoichiometry.html

A shorter way is to use a chemical factor which isn't taught anymore that I know of. A chemical factor is the ratio of molar masses.
Fe ==> Fe2O3
100 g Fe x (molar mass Fe2O3/2*atomic mass Fe) = ??