## Precalculus (6th Edition) Blitzer

We know that a combination of items occurs when the selection is made from the same group and no item is used more than one time (no repetition); also, the order makes no difference. The formula for a combination is: ${}_{n}{{C}_{r}}=\frac{n!}{r!\left( n-r \right)!}$ For example: Someone has to select 4 people from a group of 10. Thus, in this case, a combination is used to determine the number of ways to select 4 people.