Looking at the equation to calculate combinations you can see that factorials are used throughout. On the left side enter the values for the Number of Objects n and the Sample Size r. Visual demonstration on how to calculate the number of permutations and combinations when there are 5 objects and 3 are deleted.
This combination generator will quickly find and list all possible combinations of up to 7 letters or numbers or a combination of letters and numbers.
The rest of the formula is more straightforward. Pick one of the remaining two numbers two choices 4. Means factorial the product of that number and all positive integers less than it. 4 x 3 x 2 x 1 24.