How many 4 - letter code can be formed using the first 10 letters of the English alphabet, if no letter can be repeated ?

Explanation

Number of ways of filling first letter of code = 10.
Number of ways of filling second letter of code = 9
Number of ways of filling third letter of code = 8
Number of ways of filling fourth letter of code = 7
`therefore` by F.P.C total number of ways `=(10) (9) (8) (7) = 5040`.