This is a tool that calculates the number of combinations of n distinct objects taken r at a time (i.e. when the order doesn't matter).

For this calculation, it is necessary to supply the following information:

• The number of objects to choose from (n).
• The number of objects taken at a time (r).
• Whether repetition is allowed or not, i.e. whether an object can be used more than once in a combination or not (e.g. is AAA a valid combination from the letters ABC?).

Associated tool link: http://www.coggit.com/tools/number_of_combinations.html