A supermarket claims that the average wait time at the checkout counter is less than 9 minutes. Assume that the population is Normally distributed....

A supermarket claims that the average wait time at the checkout counter is less than 9 minutes.  Assume that the population is Normally distributed. We will test at 1% level of significance.

Consider

H0: mu >= 9

H1: mu < 9

A random sample of 50 customers yielded an average wait time of 8.5 minutes and standard deviation 2.5 minutes.

What is the critical value for the t-stat (the t-test statistic)?