Brain Teasers
Who Robbed the Bank?
The angry chief of police did roar,
"Who robbed the bank out of you four?"
When Al was asked, he said right then,
"The thief was Ben! The thief was Ben!"
Ben said to the policin' man,
"The thief was Dan! The thief was Dan!"
When Carl was questioned, he exclaimed,
"I'm not the one who should be blamed!"
When Dan was questioned, he replied,
"When Ben said it was me, he lied!"
If three of four suspects speak true,
And one speaks false, then who oh who?
And what if only one was frank?
But most of all, who robbed the bank?
"Who robbed the bank out of you four?"
When Al was asked, he said right then,
"The thief was Ben! The thief was Ben!"
Ben said to the policin' man,
"The thief was Dan! The thief was Dan!"
When Carl was questioned, he exclaimed,
"I'm not the one who should be blamed!"
When Dan was questioned, he replied,
"When Ben said it was me, he lied!"
If three of four suspects speak true,
And one speaks false, then who oh who?
And what if only one was frank?
But most of all, who robbed the bank?
Answer
If only one is lying, Ben robbed the bank.If only one is telling the truth, Carl robbed the bank.
Since Dan's statement was that Ben was lying, exactly one of these two must be telling the truth and the other must be lying. If Ben is telling the truth and Dan is lying, Dan must be the thief, which means that Al is lying and Carl is telling the truth. But now we have two truthtellers and two liars. So this cannot be right.
Therefore, if there is only one liar, it must be Ben. Al can only be telling the truth if Ben was the robber. If there is only one truthteller, it must be Dan. Carl can only be lying if Carl was the robber.
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