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A retail businessman had his computer stolen. The thief had to be Ruma, Jennifer, Djokovic, Tuhin, or Monisha.

When questioned, each executive made three statements:

Ruma

(a) I didn't take the computer.

(b) I have never in my life stolen anything.

(c) Tuhin did it.

Jennifer

(a) I didn’t take the computer.

(b) I have a computer of my own.

(c) Monisha knows who did it.

Djokovic

(a) I didn’t take the computer.

(b) I didn’t know Monisha before I enrolled in this school.

(c) Tuhin did it.

Tuhin

(a) I am not guilty.

(b) Monisha did it.

(c) Ruma is lying when she says I stole the computer.

Monisha

(a) I didn’t take the professor’s computer.

(b) Jennifer is guilty.

(c) Djokovic can vouch for me because he knows me since I was born.

Later each executive admitted that two of his or her statements were true and one was false.

Assuming this is true, who stole the computer?

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    $\begingroup$ The retail businessman is also a professor? $\endgroup$
    – Thomas
    May 22, 2017 at 8:05
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    $\begingroup$ @ThomasMoors I was thinking the same but I haven't been able to think of a way to change the outcome based on that... $\endgroup$
    – Brent Hackers
    May 22, 2017 at 8:22
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    $\begingroup$ @ThomasMoors - The executive named Djokovic is also enrolled in a school. Do you think he's an executive in the businessman's company and also a student at the professor's school? I'm wondering if this puzzle was copied from elsewhere with subtle changes made to avoid copyright, but not all necessary changes were spotted? $\endgroup$
    – AndyT
    May 22, 2017 at 9:43
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    $\begingroup$ @AndyT Yes, indeed, from The Moscow Puzzles: 359 Mathematical Recreations: “An elementary school teacher in New York State had her purse stolen. The thief had to be Lilian, Judy , David, Theo, or Margaret.” $\endgroup$
    – Christian Lescuyer
    May 22, 2017 at 13:12
  • $\begingroup$ Might it be good to specify that the admissions about veracity were performed under circumstances guaranteeing their truthfulness? Otherwise any or all of the executives could have lied about how many true and false statements they made, since it's already known that they don't always speak truthfully. $\endgroup$
    – supercat
    May 22, 2017 at 15:19

7 Answers 7

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First, Monisha says she is childhood friend with Djokovic, and he says it is not so. One of them is lying.
If Monisha is lying, tt is true when she said Jennifer is guilty.
If Djokovic is lying, it is true when he said Tuhin is guilty.
Jennifer OR Tuhin MUST be guilty.
Tuhin then says Monisha did it, which must be a lie and then he says that he is not guilty, which must be true.
So Jennifer must be the thief.

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It must be

Jennifer

because

Since Djokovic's b) and Mohisha's c) contradict each other, at least one of them is false. So either D c) or M b) is true. Therefore it must be Tuhin or Jennifer. However Tuhin says 'I am not guilty' and 'Monisha did it'. Since one of these must be true, Tuhin is innocent and therefore Jennifer did it.

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A different approach:

1. Tuhin
Both a and c must either be both true or both false. Since we know they gave two true statements, and one false statement, they must both be true.
Thus, we conclude Tuhin didn't do it, and he lied about Monisha doing it, so they're both innocent.
2. Ruma
We already know Tuhin is innocent, so both a and b are true.
Ruma is innocent.
3. Djokovic
We already know Tuhin is innocent, so both a and b are true.
Djokovic is innocent
4. Monisha
Since we know Djokovic was telling the truth about not knowing Monisha, we know c is false.
Thus, a and b are true - Jennifer did it.

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It's Jennifer, because only her statements do not contain two defending statements (or accusation statements).

Everyone else

has two statements that would have been both false if they are actually the thief.

Ruma

a&b (didn't take it; never stole anything)

Djokovic

a&c (didn't take it; someone else did)

Tuhin

a&b (not guilty; someone else did)

Monisha

a&b (didn't take it; someone else did)

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Based entirely on Monisha's suspicious first line:

Ruma

(a) I didn't take the computer. TRUE
(b) I have never in my life stolen anything. TRUE
(c) Tuhin did it. FALSE

Jennifer

(a) I didn’t take the computer. FALSE
(b) I have a computer of my own. TRUE
(c) Monisha knows who did it. TRUE

Djokovic

(a) I didn’t take the computer. TRUE
(b) I didn’t know Monisha before I enrolled in this school. TRUE
(c) Tuhin did it. FALSE

Tuhin

(a) I am not guilty. TRUE
(b) Monisha did it. FALSE
(c) Ruma is lying when she says I stole the computer. TRUE

Monisha

(a) I didn’t take the professor’s computer. TRUE (What 'professor'? My guess is that there were actually two computers stolen! DUN DUN DUUUN!)
(b) Jennifer is guilty. TRUE
(c) Djokovic can vouch for me because he knows me since I was born. FALSE

Which means

There were actually two crimes! However, no matter how I spin it, I still come to the same conclusion as the other answers. It seems like it has to be Jennifer who stole the computer (MAYBE TWO!).

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Everyone but one has at least two lines that mean "I didn't steel it", meaning that even if one of these statements is a lie - other proves his/her innocence. Only one claims his/her innocence only once and thus, there is a thief. Things would more complicated if there were two of such people, but this puzzle is easy at this point. My answer is same as of others.

Jenny

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Monika did it

Why:

Suppose that Ruma stole the computer. Then that would mean that A and C are lies. However, we were told that only one statement was false. Therefore, we have a contradiction and our original supposition was false. Ruma did not steal the computer. Similarly we can say that Djokovic and Tuhin did not steal the computer. Since Djokovic is lying about Tuhin being a thief, he cannot be lying about when he met Monisha.

BUT

We don't know the age difference between these people and so Djokovic could've known Monisha when she was born. He's just a friend of her family. We cannot logically conclude that the statement is a lie.

ALSO

The retail businessman is not a professor. Therefore, Monisha is telling the truth by default when she says that she didn't steal a professors computer.

Now consider that:

Everyone presumably knows who the thief is. Therefore, cannot be lying when she says Monika knows who did it. Therefore, either Jennifer does not have a computer or is the thief.

Suppose that Jennifer is lying about having a computer. Therefore, she is not the thief and Monika is the thief and lying about Jennifer stealing the computer. This works out and so it is a valid solution. You just have to accept that Djokovic is old enough to know Jennifer since she was born while working at the business.

Note to the OP: You consistently conflict between "school" and "business". Which is it?

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  • $\begingroup$ How about the retail business is that of grocery store. Let's say he hired 5 freshman to do the work. Does this still conflict with the train of thought? and who is Monika??? $\endgroup$
    – Amitabh Ghosh
    May 23, 2017 at 4:51
  • $\begingroup$ @AmitabhGhosh monika is monisha. I just cannot type. :p Umm... the issue here is that you refer to them as executives, and not students for starters. Secondly, a professor's laptop wasn't stolen (or it was at least never made clear to be that way). Finally, I cannot confirm that someone not knowing them before they came to work there indicates that they met when they both got hired. One could be a 70 year old guy bagging groceries who has worked there for 50 years straight. I might be employing odd logic, but there is in fact a secondary train of thought that deviates from your solution. $\endgroup$
    – user64742
    May 23, 2017 at 4:56

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