Trafalgar SquareSolve the number puzzles drawn on the pavement of Trafalgar Square in London. 
In London's Trafalgar Square a young man from Slovakia captures the interest of passersby with puzzles he's drawn with chalk on the pavement.
Above the puzzles were the only instructions:
When I returned the following week all nine levels were ready for the keen puzzle solvers.
Can you explain how to find the missing number in each of the grids in the photograph above? The first few levels are quite easy but then it gets difficult. The answers are available lower down this page for subscribers (except the answer to level 8 which I'm still trying to figure out).
There are some easier 'Trafalgar Square' grids below for you to complete. They contain fixed rules but randomlygenerated numbers. You can see how well you are doing by clicking the check button as often as you like as you work through the levels. If you get a level wrong try again:
Do you have any comments about this exercise? Can you do level 8 in the third photograph on this page? Do you enjoy working on the puzzles on the Transum website? Please let me know by entering a comment here.
The creators of the Trafalgar Square puzzles have made contact and kindly provided the photographs below:


Transum.orgThis web site contains over a thousand free mathematical activities for teachers and pupils. Click here to go to the main page which links to all of the resources available. Please contact me if you have any suggestions or questions. 
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Mathematicians are not the people who find Maths easy; they are the people who enjoy how mystifying, puzzling and hard it is. Are you a mathematician? Comment recorded on the 12 July 'Starter of the Day' page by Miss J Key, Farlingaye High School, Suffolk: "Thanks very much for this one. We developed it into a whole lesson and I borrowed some hats from the drama department to add to the fun!" Comment recorded on the 2 May 'Starter of the Day' page by Angela Lowry, : "I think these are great! So useful and handy, the children love them. 
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AnswersThere are answers to this exercise but they are available in this space to teachers, tutors and parents who have logged in to their Transum subscription on this computer. A Transum subscription unlocks the answers to the online exercises, quizzes and puzzles. It also provides the teacher with access to quality external links on each of the Transum Topic pages and the facility to add to the collection themselves. Subscribers can manage class lists, lesson plans and assessment data in the Class Admin application and have access to reports of the Transum Trophies earned by class members. If you would like to enjoy adfree access to the thousands of Transum resources, receive our monthly newsletter, unlock the printable worksheets and see our Maths Lesson Finishers then sign up for a subscription now: Subscribe 

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Exam Style questions are in the style of GCSE or IB/Alevel exam paper questions (worked solutions are available for Transum subscribers).
More on this topic including lesson Starters, visual aids, investigations and selfmarking exercises.
You may also want to use a calculator to check your working. See Calculator Workout skill ##.
Don't wait until you have finished the exercise before you click on the 'Check' button. Click it often as you work through the questions to see if you are answering them correctly. You can doubleclick the 'Check' button to make it float at the bottom of your screen.
Answers to this exercise are available lower down this page when you are logged in to your Transum account. If you donâ€™t yet have a Transum subscription one can be very quickly set up if you are a teacher, tutor or parent.
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Andy, USA
Saturday, August 18, 2018
"Only levels 17 seem to have a linear solution (ax + by + c = z) and are easily solved. What are the solutions to levels 8 and 9?"
Cesire Creamat, Twitter
Friday, September 14, 2018
Daniel Kardos, Slovakia
Tuesday, September 25, 2018
"Hi there,
I just found out that you uploaded those square puzzles from Trafalgar square as owner of those puzzles I found it interesting in here and I really like the idea . I solved your puzzles also. If you liked my puzzles and like to entertain people I could send to you few via email. Ps: To level eight in my puzzles you all should think bit differently."
John, Transum
Wednesday, September 26, 2018
"Hey Daniel, thanks so much for getting in touch. It was so good to see your number puzzles in Trafalgar Square. It was funny that the first time I saw your work only the first four levels had been completed. I went back the following, rainy day but the rain had made a good job of washing the chalk away so I went back a week later when I got to meet you and take the final photograph. Nobody has solved level 8 yet so it must be quite different to the rest of the levels. It would be great for you to provide more puzzles that I could include on this website. I wish you many more sunny days in Trafalgar Square. Best wishes."
Pete Davies, Blackpool
Tuesday, October 2, 2018
"For level 8. First column  second column then add 12. Took me a while! Love problems like these. Will definitely use.
[Transum: Thanks Pete, good try, but that's not the puzzle that has foxed everyone. It's the Level 8 in the THIRD PHOTOGRAPH, drawn in chalk on the pavement of Trafalgar square that we can't work out. Please share if you manage to solve that!]"
Peter Davies, Blackpool
Tuesday, October 2, 2018
"Hi. Can you tell me what the first number in the second row is for level 8 on the pavement? It is not clear on the photo. Possibly why it is challenging if people not using the correct number. Many thanks.
[Transum: Hello Pete, really pleased to know that you are joining the search for the solution to level 8 in the third photograph. It's not obvious is it? here are the numbers in the puzzling square grid:
Good Luck]"
Peter Davies, Blackpool
Wednesday, October 3, 2018
"Thinking outside the box!. For level 8 the first row numbers are in base 2. All other numbers are in base 10. If we convert the base 2 numbers to base 10 then add vertically, the missing number is 8. Hopefully correct this time!
[Transum: Your idea doesn't work adding horizontally.]"
Anil, India
Friday, December 7, 2018
"The Trafalgar Square puzzles were very tough beyond Level 3."
Corinne Brooks, Birmingham
Friday, April 24, 2020
"Nimber 8 on Trafalgar Square is 141. Simple to work out using simultaneous equations."
Kathleen, Kathleen.riley27@yahoo.ca
Tuesday, December 1, 2020
"Hi there,
I have figured out #8 but do not know how to do #9 from the Trafalgar square questions. I would appreciate a hint."