What is the mass of rust, Fe2O3, that contains 10.0g of iron?

Method 1: Figure out how many moles of Fe atoms weigh 10g. The number of moles of rust that contains that number of Fe atoms is that number divided by 2 (because there are 2 Fe atoms per molecule). Then multiply by the weight per mole of rust to get the weight of rust that has 10g of Fe.

Another method, a bit less tedious: Figure out what percent of a rust molecule is iron by weight. Then divide that into 10g to get the weight of rust required to have 10g of Fe.