看真题答案是：
If they retire at age 65:
The Tracys have a two stage horizon. 1) The first covers the six-year period until retirement. 2)
The second covers an estimated 20 years or more in retirement.
Alternatively, the Tracys could be said to have a multi-stage horizon consisting of 1) one year
during which the University payment is due, 2) five additional years of work, and 3) an estimated
20 years or more in retirement.
根据李老师的讲解，这里讲到起止时间即可拿分，即 The first stage is from age 59 to age 60.但很多真题答案里面，对这一段有补充(例如，the university payment is due.)。。。是否有必要模仿真题？
