Answer:
27,600
Explanation:
We use the following formula:-
Number of permutations of r from n
= nPr
= n! / (n-r)!
I am assuming you don't allow repeated letters in the 4 letter codes.
Take S as the first letter. Then we have 3 letters extra out of 25 letters.
Using the above formula the possible ways of doing this is
25! / (25 - 3)!
25! / 22!
= 25*24*23
= 13800.
The same number is obtained if the first letter is a T.
So the answer is 2*13800
= 27,600