Emily and Joann decide to trade jewelry collections. After the swap Joann says, "Wow! I now have 25% more pieces of
jewelry!". Emily says, "I have 15 fewer pieces of jewelry." How
many pieces of jewelry did Emily have originally?

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?