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 ?
Total number of books = 4
∴ Total number of ways = Number of arrangements of 4 books, taken all at a time = 4_{P_4}
= 4!(4−4)![∴nPr=n!(n−r)!]
= 4!0!
= 4![∵0!=1]
= 4×3×2×1
= 24
Hence, the total number of ways to arrange the books in a shelf = 24.