seven ye >> we want to figure out what bdc is. >> seven years ago he was working at an investment firm when he began tutoring his little cousin in math. when scheduling time got difficult a friend of kahn's made a suggestion. >> he said why don't you put your lectures on youtube. i said youtube is for dogs on skateboards, not mathematics. >> five years later he has produced 3,000 videos that have been viewed over 80 million times. bill gates is a big fan and donated seed money. the videos cover every level of math. >> you only have a hypotonus when you have a right triangle. it is the side opposite the right angle and it is the longest side of a right triangle. >> plus many other topics. >> that's really just saying, hey, we've had enough of you, great britain. >> they're conversational and fun, but still very thorough. >> if this does not blow your mind, then you have no emotion. >> i'm sometimes a little bit eccentric in the videos. and i think, you know, students enjoy that. this guy's, you know, kind of -- he's himself. >> the problem with our approach