My book wants me to find every unique word of "ABRAKADABRA" and since I know the formula:

Factorial of the set length divided by the product of the integer values of how many time an element has repeated itself, in this case = 11!/(5!\*2!\*2!)

Though I realise I can't rationalise this in my head. I tried painting a choice tree of something like the word "ABBA" but did not get a better understanding.