As we have already discussed, the collection of letters is important here, not the order. That means, if you have ABC in your set that’s enough. So you cant claim ABC, ACB, BAC, BCA, CAB, CBA… for combinations. These all are 1 combination of letters A, B and C.
So, from the given 4 letters (A, B, C and D), You can write the combination of 3 of those letters
ABC ABD ACD BCD
hope you have got the basic concept now.
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