Probability 101, the intuition behind martingales and solving problems with them
I like to see good math topics, but I really can't stand when the example problems are either totally fanciful or totally abstract. I'm too stupid for that. Give me an example of a practical problem in business, finance, engineering, social science, etc. that I can solve with this tool. Does the "dance party" problem have an analogue in real-world allocation and planning problems? Does a gambler gambling an <i>infinite</i> number of times have practical applications in finance? Etc.