Before Apollo 11 descended to the surface of the moon, it made one orbit at a distance of 3 miles above the surface of the moon. At one point in it's orbit, the onboard guidance system measured the angles of depression to the near and far sides of a huge crater. The angles measured 25° to the near side of the crater, and 18° to the far side of the crater. Find the distance across the crater.

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?