Question: how many moles of o2 are required to react with 24 moles of c2h6?
Answer: Well, for every two moles of ethane, it seems like you need 7 mols of O2. If you had 24 mols of ethane, well, that's 12x more moles than what you have in this reaction, right? Therefore, you'd need 12x more mols of O2 for the same reaction to go to completion, or 84 mols of O2.