Personally, I never bothered learning the formula. When I run into a problem like this, I look at it this way: (I learned this method from my high school math teacher, Mr. H. Carlisle Taylor, in 1956. It not only works, but I can even understand it !)
The first object can be any one of the 8 . For each of those ... The 2nd object can be any one of the other 7. For each of those ... The 3rd object can be any one of the other 6 . For each of those ... The 4th object can be any one of the other 5 . For each of those ... The 5th object can be any one of the other 4 . For each of those ... The 6th object can be any one of the other 3 . For each of those ... The 7th object can be any one of the other 2 . For each of those ... The 8th object has to be the 1 that's left.
