7 people receive either a black or a white ball. They can only see the color of the others balls, but not their own. Both of the colors are equally likely. They play as a team a game of guessing their own ball color.
With which strategy all of the 7 people answer correctly; and which probability of success does this strategy have?
(Sidenotes: The strategy should be made before they received the balls. And: The 7 people cannot communicate anything once they received the balls.)