66. In how many ways can three items be chosen from $n$ items?
66. First, let's calculate the number of ways to choose three items from $n$ items in a specific order. The first item can be any of the $n$ items, the second can be any of the $n-1$ remaining items, and the third can be any of the $n-2$ remaining items. In total, there are $n(n-1)(n-2)$ ways. However, with this method, each set of items (regardless of order) will be obtained exactly six times (a set consisting of items $A, B, C$ will appear in six forms: $A B C, A C B, B A C, B C A, C A B$, CBA). Therefore, the number of ways to choose three items from $n$ without considering the order of selection is $\frac{n(n-1)(n-2)}{6}$.