Does anybody know what this is and how i can use it

Kto rozmawia z clamentain przez krótkofalówke w the walking dead która śledzi lee w 4 epizodzie

Assume that there is a 4% 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 three independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?

How do we use the sumif formula (when dealing with different formats) ?