How many different four lettered codes can be formed if the first letter must be an s or a t

27,600

Step-by-step 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

correct answer:

b.(19,-2)