During one month, an accounting firm processed 51,000 invoices for Alpha Corp, 49,000 for Beta Corp, and 25,000 for Gamma Corp. Of these, 510 of the Alpha, 525 of the Beta, and 480 of the Gamma invoices had to be reprocessed for errors. a. What is the overall defect rate (dpmo) and the sigma level for each individual batch? b. For all the batches combined?