White hat black hat puzzle?

[Janet Van De Graff]

There's a circle of twenty prisoners. The jailer puts a hat on everyone's head (randomly). The hat is either black or white. The prisoner can't see their own hat, but can see everyone else in the circle's hat. They can't signal or communicate with the other prisoners in any way. The jailer asks each prisoner what color his hat is. If the prisoner gets it right, they are freed, but if anyone gets it wrong, everyone dies. The prisoners can't say anything but black or white, but are allowed to talk before the hats are put on their head. Everyone is freed. How did they do it?

Can anyone solve it?
They don't see the hats before hand :/

[alyss]

they see the colors of hat and they counted the white and black and then when they had the hats put on them they could count the other ones on the other prisoners and if the number was different then they had that color hat i think.