First, we calculate the number of cars that can be made using the given materials.
Each car needs three mirrors, we have 12 mirrors, so:
12/3 = 4 cars may be made on the basis of mirrors
Each car needs four doors, we have 12 doors, so:
12/4 = 3 cars may be made on the basis of doors
Therefore, mirrors are the excess reactants as there are more mirrors than needed to produce three cars.
