Brain Teaser Riddle: The Delivery Man
A Fun, Logic-Based Truth-or-Lie Puzzle to Test Your Wits!
Sep 10, 2008 Christopher Minster
The Riddle:
“Last week I had to make a food delivery to the State Prison. I had to make sure it was a guard who signed for it and not a convict. Now, everyone knows one thing about the men and women at the state pen: the convicts ALWAYS lie, and never tell the truth, and the guards ALWAYS tell the truth and never lie.
“I found the kitchen and there were some men working inside. When I stopped the truck, three men came over, but they were covered in flour and I could not see their uniforms: any of them might have been convicts or guards. We’ll call the men #1, #2 and #3.
“So I asked #1: ‘Hey, are you a guard or a convict?”
#1 answered my question, but just then another truck rolled by and I couldn’t hear his answer.
#2 pointed at #1 and said: ´He just said he’s a guard. He’s telling the truth, by the way.’
#3 pointed at #2 and said ‘Don’t believe him. He’s lying.’
“I thought about it for a minute, and then handed my clipboard to one of the men. ‘Sign here,’ I said. He signed, and I drove away, confident that I had done my job.”
How did the delivery man know who was a convict and who was a guard?
Hint:
Figure out what the first man said.
Solution:
They key to solving the riddle is in figuring out what the first man said. Just because you can’t hear him, doesn’t mean you can’t figure it out. The question is: “Are you a guard or a convict?” Man #1 is one of two things: guard or convict. If he’s a guard, he would tell the truth, and answer the question by saying “I’m a guard.” If he’s a convict, he would lie, and answer the question by saying “I’m a guard.” In other words, no matter what man #1 is, he said the same thing: “I’m a guard.” Those are the words that the delivery man couldn’t hear because of the passing truck.
So, #1 said “I’m a guard.” That brings us to #2, who said “He just said he’s a guard. He’s telling the truth, by the way.” Two statements. In the first statement, #2 confirms that #1 said he is a guard. That means that he is telling the truth and is therefore a guard himself. If #2 were a convict, he would have said “He just said he’s a convict.” Now that it is established that #2 is telling the truth, and therefore a guard, we can look at the rest of his statement. He said that #1 is telling the truth, so we now know that #1 is a guard as well.
As for #3, he simply calls #2 a liar, which we already know is not the case. Therefore, #3 is lying and is a convict. The delivery man gave his clipboard to either #1 or #2 to sign.
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