The next part of the “counting” part of this unit is understanding the difference between two common patterns of counting: permutations and combinations. In brief, permutations are used to count the number of **arrangements** of objects, specifically when order matters like with ranked lists, unique assignments, or sequences. Combinations are used to count the number of **groups** of objects, specifically when order *doesn’t* matter like with committees or batches.

## Today’s links

- Homework: HW 8.5 – Permutations and Combinations
- Notes: More details on the difference between these methods
- Lesson video 1: The difference between these methods (including also factorial)
- Lesson video 2: Calculating permutations and combinations (note that you can just use your calculator, instead of doing this by hand.