次のパズルを読んで問いに答えなさい。
You are about to start on a 12,000-mile (19,200km) auto trip across the country. You are told by your mechanic that your four tires definitely will last 8,000 miles (12,800km) each, but not a mile more. You can't take any chances, so you have to buy some spare and carry them with you.
問 How many spare tires do you need to buy?
――都留文科大学 中期日程 英文学科(3月8日実施)――2003年