Now lets have a look at the technical side, before going to calculate Permutations and Combinations, you should know the word Factorial.
Factorial : The factorial of a number, represented by n!, is the product of the natural numbers up to and including n
In simple words, the Factorial of the number n is the number of ways that the n elements of a group can be ordered.
So, if somebody ask you a question, how many different ways six people can sit at a table with six chairs,
you should say them its 6! = 6 × 5 × 4 × 3 × 2 × 1 = 720.
Note : We treat 0! as 1