2. Invoice errors in a billing system. In a study of invoice errors in a company's new billing system, an auditor randomly sampled 90 invoices produced by the new system and recorded actual amount (A), invoice amount (I) and the difference (or "error"), x = (A - I). The results were a sample mean = $1 and standard deviation = $124 for the differences between the actual and invoice amounts. At the time that the sample was drawn, the new system had produced 1,500 invoices. Find an estimate for the true mean error per invoice of the new system. Interpret the results.