Given : First 10 letters of English alphabet
To find : number of 4-letter codes
Since required is 4-letter code, therefore, I-place can be filled in 10 ways, H, III and IV places can be filled in 9 ways, 8 ways and 7 ways respectively (because repetition of letter is not allowed).
Hence four places can be filled in
10 x 9 x 8 x 7 ways
i.e., 5040 ways
∴ Required number of 4-letter code = 5040
