1. Suppose a code is dialed by means of three disks, each of which is stamped with 15 letters. How many three-letter codes are possible using the three disks?
2. How many three-digit codes are possible using only the digits one and zero if repetition of digits is allowed?
3. In how many different ways is it possible to seat three people in three chairs?
4. How many different ways can you arrange the five letters in the word chair if you use all five letters each time?