The word combination means selection. Suppose you are asked to make a selection of any two things from three things: a, b and c.
The different selections are: ab, bc, ac
Here, there is no reference to the order in which they are selected, ab and ba denote the same selection. These selections are called combinations.
A selection of any r things out of n things is called a combination of n things r at a time. It is denoted by nCr.
The number of all combinations of n distinct objects, taken r at a time is given by