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-5
120
The number is 26*25*24 = 15600.
There is no formula: it is a process of swapping letters in a systematic manner.
-3
If letters cannot be repeated, then there are 26 options for the first letter. There are then 25 options for the second letter. There are then 24 options for the third letter, and 23 for the fourth. This means that in total there are 26x25x24x23 options for the code. 26x25x24x23 = 358,800. Thus, there are 358,800 possible 4 letter codes.
There are 5 letters: a c e f and h.If the letters can be repeated, then there are five possibilities for each space in the four-letter arrangement. The number of arrangements then is:5*5*5*5 = 54 = 625.
A pattern
Ther are 6 letters in the word letter two of which are repeated which gives a total of four different letters.
Any 2 from 26 = 26 x 25 = 650 if ab and ba are regarded as separate, otherwise 325.
Using the [modern] Roman alphabet, there are 15600 possible passwords that are not case-sensitive.
8^5=32,768 if letters can repeat 2,880 if letters can not be repeated.