During one eight-hour shift, 750 nondefective parts are desired from a fabrication operation.

The standard time for the operation is 15 minutes. Because the machine operators are unskilled, the actual time it takes to perform the operation is 20 minutes, and, on average, one-fifth of the parts that begin fabrication are scrapped. Assuming that each of the machines used for this operation will not be available for one hour of each shift, determine the number of machines required.