Four books, one each in Chemistry, Physics, Biology and Mathematics, are to be arranged in a shelf. In how many ways can this be done?
Here, all the four books are to be arranged on a shelf. This means that we have to find the number of arrangements of the books, taken all at a time.
Now, nPn = n!
Similarly, 4P4 = 4! = 24
RD Sharma XI (2015)