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

Suppose the digit code be

Hence, the number of letters possible is

Let’s suppose any of the ten occupies place

So, as the repetition is not allowed, the number of letters possible at place is Now at and any of the alphabets have been taken. The number of alphabets left for place is and similarly the number of alphabets possible at is

Therefore the total number of letter codes