Question

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

Answer 1

Forming $4-$letter code when repetition not allowed.
Total numbers of letter $=10$
Number of letters for $1^{st}$ position $=10$
Number of letters for $2^{nd}$ position $=9$
Number of letters for $3^{rd}$ position $=8$
Number of letters for $4^{th}$ position $=7$
$\therefore$ Total number of $4-$letter code $=10\times 9\times 8\times 7=5040$