Khan Academy (English - US curriculum)
Math by grade
Eureka Math/EngageNY
Precalculus (Eureka Math/EngageNY)
Module 5: Probability and statistics
Topic A: Lesson 2: Counting rules—The fundamental counting principle and permutations
Permutation formula
Permutation formula
Sal explains the permutation formula and how to use it.
License: CC BY-NC-SA by Khan Academy