Combinations and Permutations Calculator

Question from Jessie, a student:

I need all of the 8 digit combinations using the numbers 1-20.
i.e. 1 2 3 4 5 6 7 8, 2 3 4 5 6 7 8 9 like wise
no number should be repeated in one combination.
Thanks

Find the number of combinations and/or permutations that result when you choose r elements from a set of n elements.

For help in using the calculator, read the Frequently-Asked Questions or review the Sample Problems.

Choose the goal of your analysis (i.e., to compute combinations or permutations). Enter a value in each of the unshaded text boxes. Click the Calculate button to display the result of your analysis. Choose goal   Count combinations  Count permutations  Number of sample points in set ( n ) Number of sample points in each combination ( r ) Number of combinations (n things taken r at a time)

http://stattrek.com/online-calculator/combinations-permutations.aspx