Possible Duplicate:
Combinatorics: Number of subsets with cardinality k with 1 element.
A set n has elements $\{1,2,3,\dots,n\}$. How many subsets does it have of cardinality $k$ and that contains element $1$?
Possible Duplicate:
Combinatorics: Number of subsets with cardinality k with 1 element.
A set n has elements $\{1,2,3,\dots,n\}$. How many subsets does it have of cardinality $k$ and that contains element $1$?
Hint: After you put $1$ in each possible set, you then have to fill in the remaining $k-1$ spaces from the other $n-1$ elements. How many ways can you do this?