posted 16 years ago
Welcome to the forum.
It appears that they are looking for the number of ways you can select 'k' elements from 'n' elements (without considering arrangement). This is called COMBINATION (if arrangment is considered, it is called PERMUTATION).
In your example, '22' is disallowed because once '2' is picked from {1,2,3,4}, it cannot be selected again. Since we disregard arrangement, '31' is the same as '13', and '13' is a part of your "subgroup".
Am not sure if "subgroup" is a right terminology here, but I guess it depends upon the curriculum.
- Anand
"Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away." -- Antoine de Saint-Exupery