1.Case study:

The McArthur Co. produces special vacuum cleaners for conveniently cleaning the inside of cars. About a thousand of these, with stamped serial numbers, are produced every month and stored serially in a stockroom. Once a month, an inspector does a quality control check on 50 of these. When he certifies them as to quality, the units are released from the stockroom for sale. The production and sales managers, however, are not satisfied with the quality control check since, quite often, many of the units sold are returned by customers because of various types of defect. What would be the most useful sampling plan to test the 50 units?

(Need answer of this case study plz plz)