Here's a follow up to my question here: Probability combination contains no repetition of letters
All the combinations of $n$ letters, $r$ together, repetitions permissible, are written down. Find the chance that a combination selected at random will contain no repetition. ($n \ge r$)
The answer in the back of my book is as follows:
Possible combinations $= R_r^n$. Favorable combinations $=C_r^n$. Chance $= C_r^n \div R_r^n$.
However, I don't understand what $R_r^n$ is, and I couldn't find where the notation $R_r^n$ is introduced in the book. Could anyone explain what $R_r^n$ is, and how $R_r^n$ is the correct choice for possible combinations here?
