From Khan Academy:
1. introduction to Present Value - http://www.youtube.com/watch?v=ks33lMoxst0&list=PL9ECA8AEB409B3E4F&index=3&feature=plpp_video
2. http://www.youtube.com/watch?v=4LSktB7Pk_c&list=PL9ECA8AEB409B3E4F&index=4&feature=plpp_video
3. http://www.youtube.com/watch?v=3SgVUlEcOBU&feature=fvwrel
4. http://www.youtube.com/watch?v=6WCfVjUTTEY&feature=fvwrel
