Simple combinatorics formula i am having some trouble trying to understand this simple formula: 
$\frac{n!}{\left(n-r!\right)r!}$ 
Even though i am able to solve exercises easily using this formula i can't understand it. How was this formula derived? Is it possible to understand the formula or is it just the result of some certain mathematical work? 
The part that i don't understant is how does diving the possible combinations by the redundancies result in the possible number of arragements of something ignoring order.

In short, it's a rearrangement and simplification of a binomial coefficient. http://mathworld.wolfram.com/BinomialCoefficient.html
