Admission to a concert is $20. It costs an additional $7.50 for each T-shirt you bought. You spent a total of $57.50 at the concert. How many T-shirts did you purchase at concert?

Assume that there is a 11% rate of disk drive failure in a year. a. if all your computer data is stored on a hard disk drive with a copy stored on a second hard disk drive, what is the probability that during a year, you can avoid catastrophe with at least one working drive? b. if copies of all your computer data are stored on four independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?