Probability and Expectation

In China, lots of excellent students who are good at maths take an active part in various maths contests and the best six senior high school students will be selected to form the IMO National Team to compete in the International Mathematical Olympiad. In the past ten years China's IMO Team has achieved outstanding results — they have won the first place almost every year.

The author is one of the senior coaches of China's IMO National Team, whose students have won many gold medals many …