Scotland Yard is trying to catch an important London drug cartel. They tend to do large trades worth more than 1 million great British pounds in several locations around London - mainly parks and cemeteries.

Through a network of informers the police have been able to map all the locations where they meet - and you can assume that drug dealers only meet at these locations.

enter image description here

And here is a link for the map with the location names.

The chief of Scotland Yard has assigned a very limited budget to catch this cartel. So each day they can dispatch, at most, a single Anti-Drug unit.

The problem with catching these guys is that the cops do not know, ex-ante, at which place they are planning to meet. Usually a message between buyers and sellers is sent before the next meet. During the past year, cops have managed to intercept few messages, but the messages only contain the time of the next meet and never the location. Out of the messages they have intercepted, some of them Scotland Yard, managed to trace to a location by randomly choosing a location to go. Here are the messages traced which cops have managed to find the meeting location (through the informers network):

enter image description here

The meetings always occur within a 24-hour window from the time the message is intercepted and always occur at the time settled on in the message.

Now the Scotland Yard, decided to hire a consultant, YOU! Today (9-Sep-2016) they have just intercepted this message: " Meet at 9 p.m."

Can you determine where in London the meeting will occur?

Here is the CSV format:

Intercepted on: ,Message,Meeting Location
03/01/2016,Meet at 7 p.m,Westow Park
17/01/2016,Meet at 4 a.m.,Shepherds Bush
19/02/2016,Meet at 10 a.m.,Figge's Marsh
24/02/2016,Meet at 9 a.m.,Wandsworth Common
27/02/2016,Meet at 2 a.m.,Talacre Gardens
02/03/2016,Meet at 5 a.m.,Darthmouth Park
05/03/2016,Meet at 11 p.m,London zoo
13/03/2016,Meet at 11 p.m,Golders Hill Park
17/03/2016,Meet at 11 a.m.,Croydon Cemetery
24/03/2016,Meet at 10 a.m.,Figge's Marsh
25/03/2016,Meet at 1 p.m,Dulwich Park
30/03/2016,Meet at 2 a.m.,London zoo
13/04/2016,Meet at 10 p.m,Shepherds Bush
18/04/2016,Meet at 8 a.m.,Shillington Gardens
25/04/2016,Meet at 2 p.m,Grangewood Park
26/04/2016,Meet at 5 p.m,Ruskin Park
01/05/2016,Meet at 10 a.m.,Wandsworth Cemetery
14/05/2016,Meet at 8 p.m,Hyde park
18/05/2016,Meet at 12 p.m.,Westow Park
25/05/2016,Meet at 8 a.m.,Fishponds Playing Field
29/05/2016,Meet at 10 p.m,Roundwood Park
31/05/2016,Meet at 4 a.m.,Darthmouth Park
13/06/2016,Meet at 2 a.m.,Primrose Hill
10/07/2016,Meet at 5 p.m,Meetpoint Hampsted
28/07/2016,Meet at 8 a.m.,Grangewood Park
16/08/2016,Meet at 9 a.m.,Grangewood Park
24/08/2016,Meet at 4 p.m,Roundwood Park
25/08/2016,Meet at 10 a.m.,Greendale Playing Fields
01/09/2016,Meet at 9 a.m.,Greendale Playing Fields
03/09/2016,Meet at 2 a.m.,Figge's Marsh
  • $\begingroup$ This seems a little unfair, only people in Europe can read these dates. +1, nice map, good organization too. $\endgroup$
    – John
    Sep 9, 2016 at 16:34
  • $\begingroup$ It's just dd/mm/yyyy. A little different, but easily figured $\endgroup$ Sep 9, 2016 at 16:36
  • 1
    $\begingroup$ You may want to put the meeting times and locations in a flat/text format if there's a pattern there. It will make it easier for someone to work with. $\endgroup$
    – John
    Sep 9, 2016 at 17:05
  • 1
    $\begingroup$ Do we have to solve this riddle before 9pm today (UK time)? $\endgroup$ Sep 9, 2016 at 17:48
  • 6
    $\begingroup$ @John I think you mean "only people in the whole world, other than North America" $\endgroup$
    – ffao
    Sep 9, 2016 at 18:48

2 Answers 2


They are most likely going to meet at...

Croydon Cemetery

How I came to this conclusion?

Step 1: Try to find out if there is a strong co-relation between the day of the week and active area.

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Thursday & Friday is always South Side of the river.

Step 2: Try to find if on the South Side of the river, there is a co-relation with the time of the deal.

enter image description here

It’s all the east side locations that show up post noon.

Step 3: See how deals have happened in these areas post noon.

enter image description here

There has been a deal at all locations except Greendale Playing Fields and Croydon Cemetery

Step 4: Eliminate.

enter image description here

Peddlers had two deals recently in Greendale Playing Fields and non at Croydon Cemetery since March. So it's show time.

  • $\begingroup$ Upvote for the North River/South River, that is a piece of the solution. However your solution is not correct. $\endgroup$
    – phdstudent
    Sep 9, 2016 at 20:10
  • $\begingroup$ i should have treated it as a riddle instead of my C# program :) $\endgroup$
    – WeShall
    Sep 9, 2016 at 20:14
  • $\begingroup$ @volcompt: do i need to consider related but not specified details like co-ordinates or connecting routes or distance between locations ? Any other hints !? $\endgroup$
    – WeShall
    Sep 10, 2016 at 18:09
  • $\begingroup$ No need to consider coordinates nor connecting routes. The only thing you need to know is the time of the meeting $\endgroup$
    – phdstudent
    Sep 11, 2016 at 0:00

If you look at the map, it broadly resembles 2 clocks with hourly positions north and south of the river.

I labelled locations North:1, North:2 etc.. according to their hour on the clock.

I separated the data into north and south, and then through a little trial and error discovered the relationship that that the location is related by the formula:

$$ \mod_{12} \left ( 13 - \mod_{12} ( hour + month + weekday ) \right ) $$

(where weekday is 1=Sunday, 7=Saturday)

For example the first item: 3/1/2016 at 7pm is:

$$ \mod_{12} \left ( 13 - \mod_{12} ( 7 + 1 + 1 ) \right ) = 4 $$

(and South:4 is Westow Park)

This formula predicts every meeting location except 13/3/16 which is measured as Golders Hill Park = 11 but the formula suggests it is actually Hempstead Cemetery = 10.

This suggests that the for the next meeting on 9/9/2016 the location is:

$$ \mod_{12} \left ( 13 - mod_{12} ( 9 + 9 + 6 ) \right ) = 1 $$

So the meeting place is either Dartmouth Park or Ruskin Park.

Haven't quite figured out South/North yet...

  • $\begingroup$ Try to combine your answer with the first part of WeShall's answer and give them credit if you do so. I think it will help finish your answer :) $\endgroup$
    – AHKieran
    Jun 3, 2019 at 8:41
  • $\begingroup$ Yeh I looked at that but I didn't agree with it, or at least it didn't seem to help so will try again later $\endgroup$
    – Attack68
    Jun 3, 2019 at 8:42

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