An Automobile Rustproofing Company Claims That Their Methods Protect Cars $\bar{x}$

Short Answer

An automobile rustproofing company claims that their methods protect cars for 60 months. This hypothesis is tested against the alternative that the protection lasts for less than 60 months. A random sample of 100 cars produces an average time of 57 months with a standard deviation of 15 months.
-Find the value of $\bar{x}$ necessary to reject the null hypothesis.