Video sets up and solves the following brain teaser: Three people with infallible logic are in a room together. Each has a number painted on their forehead. The numbers are unique, greater than zero, and one number is the sum of the other two. The first person has 20 on his forehead and the second person has 30 on his forehead. When asked what number is on their forehead, each of the two people say they don't know. The problem question is: What is the number on the forehead of the third person, and why? [12:08]