Question

Explain combinations.

Answer 1

A combination is a way of selecting items from a collection, such that the order of selection does not matter.
A formula for the number of possible combinations of r objects from a set of n objects. This is written in any of the ways shown below.
$\left(\begin{array}{cc}n& \\ r& \end{array}\right)or{}_n{C}_{r}orC(n,r)oroccasionally{C}_r^n$
For eg. say you have 4 balls numbered 1,2,3 and 4. The possible combinations are:

${}_n{C}_r=\frac{n!}{r!(n-r)!}$