Nice Puzzle

Same problem in disguise:
http://www.able2know.com/forums/viewtopic.php?p=1128936#1128936

Yups Mark Thats Right .....

But can you tell me how maximum 2 questions suffice this solution?

Quick Answer


For those who want a quick solution

Phase 1.

Label the 3 thieves as A, B, C

They can say the truth +, false - or Random X …. So possibilities are



Ask First question to A as "Do I have more chance with B for truth than C"

Possibility 1: If answer is Yes then



would be valid combinations to look into

Look carefully at C's column, X (random) is absent under all circumstances.

So either C is a liar or a truth-teller but definitely not a lunatic


Possibility 2: If answer is No then



would be valid combinations to look into

Look carefully at B's column, X (random) is absent under all circumstances.

So either B is a liar or truth-teller i.e. non lunatic



Phase 2

After the above question one non lunatic person is finally identified may it be C or B.

Lets call him the "Key"

Case 1:

Conside if "Key" is a liar i.e blood group B Rh+

Now Ask him 2nd question that "If I ask to the other non AB+ blood group man among 3 of you that "You 3 robbed the bank" what would he say?"

"You 3 robbed the bank" is a fact so its truth value is Yes

But the other non AB Rh+ person will be truth teller (bcoz "Key" is the liar).

Hence intoxicated the other non AB + person would admit the crime and say "yes"

But bcoz "Key" is a liar he would twist the reply and say "No"

Case 2:

Lets say if "Key"is a truth teller i.e blood group other than AB & B +
Upon asked the same question "If I ask the other non AB+ blood group man among you 3 that "You 3 robbed the bank" what would he say?" to Key …

"You 3 robbed the bank" is a fact so its truth value is Yes

But the other non AB + person will be the B + blood so he will be a liar.

Hence unaffected by the drug he would be clever enough to say "No"

Now as "Key" is a truth teller. So he straight conveys the others answer as "No"

So after the 2nd question under any circumstances the lawyer is going to get an answer "No" from the "Key" person that is opposite of the factual truth ("You 3 robbed the bank"), negation of which will prove the fact and hence the chemical lie detector would not go in vain and all parameters met the thieves are caught under their own acceptance of crime.

Okay...BUT

1.If it's simply ineffective on them, they could lie or they could tell the truth. It would have to have an effect on them (opposite of its intended effect) to make them lie without fail.

2. I find it hard to believe that the criminals know each others' blood types and how the drug will effect them Or that a jury would buy this argument, for that matter :wink:

1. Normally with fear of getting caught, every criminal would lie ... although B Rh+ people aren't affected with the drug but B Rh+ CRIMINALS will defintely lie.

2. You got me there

Sure, they'd lie about robbing the bank under normal circumstances. But a wylie criminal might just tell the truth to ruin your plan. More to the point, you won't be able to convince the jury of this:


To them, it's possible that the group of three is innocent and the "NO" is not the result of negation. We know a priori that they are guilty, but an (unbiased) jury does not. To them it would seem that the person with B positive blood type may not have incentive to lie and the whole argument unravels.

In effect, you would have proved that if you assume they did it, you can show they are guilty and if you assume they didn't do it, you can show they are innocent :wink: