Some videos on permutation and combinations
https://www.youtube.com/watch?v=hJRXKq2GEo8&hd=1
https://www.youtube.com/watch?v=qaFNhwNBY3k&hd=1
A somewhat longer video
https://www.youtube.com/watch?v=QCq6qRJCTj8&hd=1 by Pushpinder Gill