The mean gross incomes of certified welders are normally distributed with the mean of $20,000 and a standard deviation of $2,200. The ship building association wishes to find out whether this number is still accurate. You take a sample of 45 welder's salaries and find the average to be $21,500. Using a significance level of 0.10, has there been a change in the mean gross annual income of certified welders?
a. State the null and alternate hypothesis
b. What is the level of significance?
c. What is the critical value?
d. Compute the value of the test statistic
e. What is your decision regarding the null hypothesis? Interpret the result.