The flu, by comparison, has an average incubation period of just two days.

相比之下，流感的平均潜伏期 只有短短两天的时间。

Because some students attend college and others don't, let's just assume an average period of 15 years spent in school.

因为有些学生会上大学，有些则不会，我们就假设平均在校时间为 15 年。

If you work for eight hours a day, five days a week, for an average period of 45 years, that amounts to 3900 days.

如果你每天工作 8 小时，每周工作 5 天，平均工作 45 年，这相当于 3900 天。