What if you only arrange r objects chosen from n? You don't use all n.
Example: 5 books, pick 3 to arrange on a shelf. How many ways?
First slot: 5 choices. Second: 4. Third: 3. Total: 5 × 4 × 3 = 60.
This is called a partial permutation or r-permutation of n. Next, I'll give you the formula.