Tech$-$Smart Computer Company recently discovered a defect in the hard disk installed in its model R$24$ computer ThE hard Disk head in these units retracts too violently whenever the computers are turned off. As a result, the hard disk are destoryed after the computer is turned on and off approximately $500$ times. Tech$-$Smart has sold $4,000$ model R$24$ computer and discovered that $20$ percent$($or $800)$ use their computers in businessthay operates $24$ hours per day.