For the example you can calculate 10. The length is the potential of the field. If you have to solve by hand keep in mind that for each factorial you start with the main number given and then multiply it by the next smallest number and so on until you get down to 0.
Pick one of the remaining two numbers two choices 4.
Apart from the three winning numbers there are seven other numbers that can be chosen for the fourth number. N is the total number of elements in the set. Pick one of the remaining two numbers two choices 4. Choose Count combinations as the analytical goal.