Q. Find the number of ways three letter "words" can be chosen from the alphabet if none of the letters can be repeated?
Solution: There are 26 ways of choosing the first letter, 25 ways of choosing the second letter, and 24 ways of choosing the third letter; thus there are 26.25.24 = 15,600 ways of choosing the three letters. Using the formula of permutations, we have ^{26}P_{3} = 26.25.24 = 15,600