A combination is a mathematical technique that determines the number of possible arrangements in a collection of items where the order of the selection does not matter.

**Formula to calculate combination.**

- n represents the total number of items
- r represents the number of items being chosen at a time.

To calculate a combination, you will need to calculate a factorial. For example to calculate the factorial of 4, you would multiply all of the positive integers equal to and less than 4. So, 4! = 4 x 3 x 2 x 1. By multiplying these numbers together, we can find that 4! = 24.

Example:

There are ten new movies out on Netflix this week on DVD. Rose wants to select four movies to watch this weekend. How many combinations of movies can she select?

n = 10

r = 4

Therefore, Rose can select 210 different combinations of new-release movies this week.