Combinations

For $n$ different objects, if we select $r$ objects out of these $n$ objects which the order of the selected objected does not matter, then the number of ways is
\begin{equation*}
C^n_r = \frac{n!}{r!(n-r)!}
\end{equation*}

Same Topic:

Complementary Events Experimental Probability Mutually Exclusive Events Theoretical Probability