Exponential growth: the world population was 2010 about 7 billions; the yearly population growth is just below 2%. assuming r=2% being stable:
how much time did elapse since the people were only 3.5 billion?
[type the nummerical value rounded to full years ]

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?