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37.8 is the answer

change 20 kg to 20000 g, multiply it by (1 mol/27g) youll get 740.704 mol of Al

then you know you need half as many moles of al2o3 as you do al so divide 740.704 by 2. youll get 370.37 mol of al2o3 are needed to get the 20 kg of al

next you multiply 370.37 mol by the molecular mass which is 102. that gives you your answer in grams its 37.8g