Carlie needed a personal computer $($PC$)$ for school but her budget was tight, so she decided on a used machine. After some searching, she was down to two possibilities. One she had found in a national franchized computer store, where she could purchase the PC for $$800$ with a one$-$year guarantee. But she was intrigued by a second possibility: a local charity store was offering a used machine that had been donated $($exactly the same model as in the store$)$ for $$500.$ Carlie talked to a computer repair specialist, who said the principal problem with this model of PC was the hard drive, which was prone to failure: he estimated that half of the PCs of this model and age would require a new drive. He offered to test the computer and check the condition of the drive for $$60.$ If the drive was no good, a new drive could be fitted for $$300$ and would be guaranteed for one year. He added a qualification that his testing was not perfect: about $25\%$ of drives that passed his testing failed soon afterwards requiring a new drive $($$$300)$ plus a service charge of $$100$ for diagnosing the problem. What should Carlie do$?$