Two trains left the station at the same time, one heading due east at 40 miles per hour and the other heading due west at 50 miles per hour.

how far apart were they after 4 hours?

don't tell me the answer just tell me how to do it

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?