Anhydrous aluminum oxide can be reduced to aluminum according to thisa chemical equation: 2 Al2O3 (s) ---> 4 Al (l) + 3 O2 (g) Calculate the mass of aluminum oxide reacted if 91.4 g of Al was produced and the percent yield was found to be 89.3%

91.4/0.893 = 102.3 grams produced at 100% yield.

Using the 102.3, use conventional stoichiometry to solve for Al2O3 initially. Here is a sample problem I posted.
http://www.jiskha.com/science/chemistry/stoichiometry.html