Hi i have been studying permutation and combination i fell into a problem. if You have a set of letters to choose from = {A,B,C,D}, and you must choose 3 of them, but no immediate repetition of a letter.
Example {A,B,A}, {C,D,C}, etc is allowed.
What formular, how should i solve this problem?
Thanks in Advance
For the first item in the resulting set we have $4$ choices.
For the second item in the set we have $4-1 = 3$ choices as we cannot repeat the previous letter.
Finally for the third item in the set we have $4-1= 3$ choices as again we cannot repeat the previous letter.
Multiplying the number of choices (using the rule of product) we have $4*3*3 = 36$ solutions.