How many ways can 3 books be placed on a shelf if chosen from a selection of 7 different books?

There were 25 students who answered to a survey about sports. four-fifths of them like football. how many students like football?

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?

If f(x) = 4x - 3 and g(x) = x + 4, find (f - g) (x)