A collection of mysterious and unsolved math problems, also known as “The Millennium Problems” are 7 extremely challenging and complex mathematical problems selected by the Clay Mathematics Institute in 2000. Solving any of these problems would not only advance our understanding of mathematics but also earn the solver a prestigious $1 million prize. To date, only one of these problems (The Poincaré conjecture) has been solved, leaving six intriguing and unsolved mysteries awaiting discovery.
The answer is 4 and I refuse to show my work. Money please. ;)