A tire company claims that the lifetimes of its tires average 50500 miles. The standard deviation of tire lifetimes is known to be 5500 miles. You sample 150 tires and will test the hypothesis that the mean tire lifetime is at least 50500 miles against the alternative that it is less. Assume, in fact, that the true mean lifetime is 50000 miles. Compute the power of a test At what level should the test be conducted so that the power is 0.85.