Tech$-$Smart Computer Company recently discovered a defect in the hard disks installed in its model R$24$ computer. The hard disk head in these units retracts to violently whenever the computers are turned off. As a result, the hard disk are destroyed after the computer is turned on and off approximately $500$ times. Tech$-$Smart had sold $4,000$ model R$24$ computers nationwide. The marketing department at Tech$-$Smart contacted most of the $4,000$ owners of the model R$24$ computer and discovered that $20$ percent $($or $800)$ use their computers in businesses that operate $24$ hours per day. These customers never turn their computers off; therefore $,$ the defect should not damage their hard disk units.