不明白这个最小收益率是怎么算出来的？

问题如下：

An investor currently has a portfolio valued at $700,000. The investor’s objective is long-term growth, but she will need $30,000 by the end of the year to pay her son’s college tuition and another $10,000 by year-end for her annual vacation. The investor is considering three alternative portfolios:

Using Roy’s safety-first criterion, which of the alternative portfolios most likely minimizes the probability that the investor’s portfolio will have a value lower than $700,000 at year-end?

选项：

A.Portfolio 1 B.Portfolio 3 C.Portfolio 2

解释：

The investor requires a minimum return of ($30,000 + $10,000)/$700,000, or 5.71%. Roy’s safety-first model uses the excess portfolio’s expected return over the minimum return and divides that excess by the standard deviation for that portfolio:

Safety-first ratio=[E(R_p)-R_l]/ σ_p,

where E(R_P) is the expected return of portfolio P,

R_L is the minimum return required by the investor, and

σ_p is the standard deviation of returns of portfolio P.

The portfolio with the highest safety-first ratio minimizes the probability that the investor’s portfolio will have a value lower than $700,000 at year end. The highest safety-first ratio is associated with Portfolio 3: 0.3768.