Answer: Just 2 times of weight: First time: Divide 9 coins into 3 parts, each with 3 coins (3; 3; 3). Place the first and the second parts on the scale, if somewhat lighter, the part contains counterfeit copper, if these two parts are equal, the counterfeit is in the third part.

2nd time: After the 1st weighting above, the part containing the fake copper was identified. This part has 3 coins, of which only 1 fake. Put any 2 coins on two weighing plates, if the dong is lighter then that is a fake, if the two coins are equal, the other is a fake.

Answer: Because a prisoner is always telling the truth and the other person always says so, when the prisoner asks through the prison guards in turn, the answer is always wrong. The question is: Please ask him where is the exit and tell me what his answer is? Prisoners will always get the wrong answer and go in the opposite direction to the answer.