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Jenny works in an office with four other colleagues and someone has been using her milk. She finds this very annoying as she likes her mid-afternoon tea and she simply can't enjoy the second half of her day if she has to drink black tea instead.

Deciding that enough is enough Jenny decides to do something about it, she's suspicious of all of her four colleges Liz, John, Sarah and Bill so she decides she's going to solve the crime once and for all.

Her first course of action is to make a note of which days her milk is used and which days her coworkers are in the office.

  • Monday - JBS - THEFT!
  • Tuesday - JLBS - THEFT!
  • Wednesday - JLBS - THEFT!
  • Thursday - LBS - No theft
  • Friday - LB - No theft

Next, deciding to forsake her regular duties for the sake of office security she follows various members of the office for a day over the next two weeks. Alas the thief evades detection.

  • Monday - Sarah
  • Tuesday - Liz
  • Wednesday - John
  • Thursday - Bill
  • Friday - Liz
  • Monday - Sarah
  • Tuesday - Jenny has to attend a meeting and fails to follow John all day
  • Wednesday - Liz
  • Thursday - Liz
  • Friday - Bill

To her interest the thefts occur on the same days each week (Monday, Tuesday and Wednesday).

By the second Friday she's at her wit's end, she's still not caught the thief - is she doomed to a lifetime of black tea!?

She explains her dilemma to her friend one evening who (also being British) realises the seriousness of the situation. Pouring over Jenny's copious notes it looks like a lost cause until her friend smiles. She knows who the thief is!

Who is responsible for the milk thefts?

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    $\begingroup$ Why should we assume that only one person steals milk? I mean, other than the fact that assuming this gives a unique solution to the problem. Should it be stated, or are you happy to leave that to the solver? Especially given the solution, it's not clear to me why there shouldn't be one person who chooses to steal milk only on Monday, and another who chooses to steal only on Tuesday. $\endgroup$
    – Steve Jessop
    Nov 10, 2014 at 17:41
  • $\begingroup$ I designed the question assuming there only one thief but feel free to prove me wrong! $\endgroup$
    – Liath
    Nov 10, 2014 at 19:02
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    $\begingroup$ What prevents (for example) John from stealing on Monday, Sarah on Tuesday, and Bill on Wednesday? They were all there on the proper days, and were innocent when followed. If you don't restrict to one thief, you can always assume the thief for a given day was someone who wasn't followed. $\endgroup$
    – TheRubberDuck
    Nov 10, 2014 at 20:51
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    $\begingroup$ "Pouring over Jenny's copious notes..." I see what you did there. :-) $\endgroup$
    – AndrewJacksonZA
    Nov 10, 2014 at 21:22
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    $\begingroup$ Was this inspired by this workplace.se question? $\endgroup$
    – reo katoa
    Nov 10, 2014 at 22:17

6 Answers 6

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it's Bill

They only steal it on the days John is in though; that's why they weren't caught red handed.

If she was going to waste two weeks why didn't she just watch the fridge?

Based on the first set of clues you would assume it was John because it's only stolen on the days he's there. However she followed him on Wednesday so we know it's not John. She also followed the other two besides the culprit on the days it was taken. So the one who stole it the only one who isn't followed on a day they where it is stolen. Her friend noticed this pattern as well. The culprit must be trying to implicate John based on attendance alone.

However, the culprit should not be punished; he seems to work very hard as does Liz. Jenny apparently can waste 2 weeks following people around. The company should probably fire Jenny and provide free milk for the thief.

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    $\begingroup$ I'm REALLY glad you got that - I was paranoid that I'd messed up somewhere... $\endgroup$
    – Liath
    Nov 10, 2014 at 15:02
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    $\begingroup$ Two weeks is a small price to pay for the sanctity of afternoon tea! $\endgroup$
    – Liath
    Nov 10, 2014 at 15:04
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    $\begingroup$ @Liath added logic. Like the puzzle though. I almost answered with the wrong person but then realized that the second set of information would have been useless. $\endgroup$
    – Bozman
    Nov 10, 2014 at 15:14
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    $\begingroup$ "If she was going to waste two weeks why didn't she just watch the fridge?" Because the fridge has a straight line of sight to the managers office, so hanging around the fridge all day would of course be noted by him. $\endgroup$
    – David Mulder
    Nov 10, 2014 at 15:45
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Actually we only need 4 Lines of information to solve this:

Monday    - JBS  - THEFT!
Wednesday - JLBS - THEFT!

So we know it is either John,Bill or Sarah! And we know the Thief is stealing on Mondays and Wednesdays.

Monday    - Sarah
Wednesday - John

So we know it is not John or Sarah, which leaves

Bill

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    $\begingroup$ I like this approach, its about cutting out the story and uncovering the facts $\endgroup$
    – skv
    Nov 10, 2014 at 15:28
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    $\begingroup$ Wait, where did "Biz" come from? $\endgroup$
    – David Mulder
    Nov 10, 2014 at 15:46
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    $\begingroup$ Bill and Liz had a baby who also has an affinity for other people's milk. $\endgroup$
    – Bozman
    Nov 10, 2014 at 15:51
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A binary table solves this in a pretty obvious way:

I merged the 2 weeks duration for the sake of simplicity.

| MO | TUE | WED | Theft and Observation
|  1 |  1  |  1  | Theft
|  1 |  0  |  0  | Sarah
|  0 |  1  |  0  | Liz
|  0 |  0  |  1  | John
|  0 |  0  |  0  | Bill

As you can see Only

Bill

was not stalked during the days of the theft. Which makes him the suspect of this serious crime :(

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  • $\begingroup$ ah I'm sorry I am new to this how does one make the block qoute visible on hover? $\endgroup$
    – cptnk
    Nov 10, 2014 at 16:01
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    $\begingroup$ added it for you. its just >! $\endgroup$
    – Bozman
    Nov 10, 2014 at 16:02
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    $\begingroup$ @cptnk For future reference, there's a "?" icon to the upper right of your text box where typing your answer with some help, and an "advanced help »" link with the answer you were looking for. $\endgroup$
    – Tim S.
    Nov 10, 2014 at 17:53
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    $\begingroup$ Or, click "edit" on an existing answer and look at the codes used. $\endgroup$
    – Mr Lister
    Nov 10, 2014 at 22:43
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Bill is the only one present on all the days the milk was stolen (M,T,W) AND wasn't followed by Jenny during the theft period (only followed on TH, F)

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Based on the information provided, and some information not provided, there are two possible outcomes of the single thief theory. The first as deduced by others with the cross reference of data provided.

Bill. The second, who was not considered, has all the opportunity, and none of the scrutiny. This person has been stated to have easy access to the fridge. And is also likely to be have been in the office on each of the days the theft occurred. Since no one was observed committing the crime, this person could not be ruled out as a suspect. The manager could easily helped themselves to the milk. If they were a good manager they would have noticed two things: 1 someone was taking the milk, 2 that action had cost him / her over 40 man hours. So with that observation, I would not put it past the manager to have a little refreshment.

I need to slow down on the murder mysteries.

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This question appears overly easy.

In following her colleagues around for the first three days, it is established it is not Sarah, or Liz, or John. So, it must be Bill. All other information is superfluous.

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    $\begingroup$ This has been pointed out before years ago by other users. $\endgroup$
    – gabbo1092
    Nov 29, 2018 at 19:51

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