Exercise $1($Try it yourself$)$

During one $8-$hour shift, $750$ nondefective parts are desired from an operation. The standard time for the operation is $15$min. Because the machine operators are unskilled, the actual time it takes to perform the operation is $20$min and, on the 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 $1hr$ each shift, determine the number of machines required.

$41$