You can choose any of the 80 books first then any of the remaining 79 next, 78 third and, finally, 77 last. There are (80)(79)(78)(77)= 37957920 choices. That is the same as Subhotosh Khan's \(\displaystyle \frac{n!}{(n-r)!}\) for "combinations". Since the bag doesn't care what order the books are put in, we need to divide by 4!= 24 to remove the 24 different orders in which the four books can be put in. That gives \(\displaystyle \frac{37957920}{24}= 1581580\) choices. Again that is the same as Subhotosh Khan's formula for "permutations".