A sales manager of a hardware store monitors sales trends in the plumbing section by random sampling 12 sales invoices at the end of each month. Historically, the monthly average sales invoice has been stable at $120. Last month the sample gave an average value of $112.85 with a standard deviation of $20.80. Does this evidence support at the 5% level of significance the contention that the average long term sales invoice value has changed? (Assume that sales invoice amounts are approximately normally distributed.)