Which property would allow you to use mental computation to simplify the problem 27 + 15 + 3 + 5?

Can you me find the missing length. i attached an image.

I’m really confused and need your assist me with this question i’ve never been taught

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?