A hiker is 15 feet above sea level. (Sea level is 0 feet.) A diver is 20 feet below sea level. Find the difference in feet, between the hiker and diver. I NEED HELP FOR MY QUIZ!

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?