How many 4letter words can you form from the alphabet AZ if

How many 4-letter “words” can you form from the alphabet A,...,Z if the letters are in alphabetical order and

(a) repetitions are not allowed, e.g., BEFS, (b) repetitions are allowed, e.g., BFFS.

We know that there 26 letters in alphabets

(a)

We have to find number of 4 letter words

since, repetition is not allowed

so, each 26 letters cannot be in each four letters

if first letters can have 26

second letters =25

third letter =24

fourth letter =23

so,

number of 4 letter word = 26*25*24*23

number of 4 letter word=358800..............Answer

(b)

now, we are given repetition are allowed

so, each 26 letters can be in each four letters

if first letters can have 26

second letters =26

third letter =26

fourth letter =26

so,

number of 4 letter word = 26*26*26*26

number of 4 letter word=456976..............Answer