: An arrangement of $r$ :objects chosen from $n$ objects, in which the $n$ objects are distinct, there is no repetition of the objects, and order is important.

$$ : An arrangement of $r$ :objects chosen from $n$ objects, in which the $n$ objects are distinct, there is no repetition of the objects, and order is not important.

$$ A measure of a particular event happening comparted to all events in a sample space.