In how many ways can seven books be arranged on a shelf?

Question: In how many ways can seven books be arranged on a shelf?

Solution:
Number of ways in which the first book can be placed = 7
Number of ways in which the second book can be placed = 6
Similarly,
The total number of ways in which seven books can be arranged on a shelf = 7 × 6 × 5 × 4 × 3 × 2 × 1
= 7!
= 5040