Twenty percent of the engines manufactured on an assembly line are defective. Suppose engines are randomly selected one at a time and tested. Given that the first two engines tested were defective, what is the probability that at least two more engines must be tested before the first nondefective is found

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?