Derek has $20 to spend on used books. Hardcover books cost $5 each and paperbacks cost $2 each. Complete the equation which determines the number x of hardcover books and the number y of paperback books he can buy.

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?