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'Zin Obelisk' printed from http://nrich.maths.org/

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We had a number of correct solutions including some from students from Our Lady's G.S. Newry, several from students from Marist College in New Zealand (Kate and Lauren, Aimee and Hannah, Jessica and Alesha, Helen and Maiya), Ben, Alex, Rob and Harry from the Royal Latin School, Niall, Tyler, Daniele, Michael and Thomas from St.Margarets C.E.Primary School, Ben, Beth, Caitlin, Connor H, Connor L, Flora and Harry from Maynards Green Primary School, Kai, Harry and William from Herbert Strutt School, Bandhagi from The Garden International School and Charlotte, China, Kyle, Viva, Dannielle, Jordan and Emily, all from Kavanagh College.

The Maths Challenge Group from Colyton Grammar School sent a very clear explanation of how they arrived at the correct result:

As a start, we organised the cards into categories:

Facts about the obelisk
Facts about time measurement
Facts about workers

We also found that there were a number of cards that were irrelevant to the solution.

Given the dimensions of the obelisk, we found that its volume is 100 x 50 x 10 = 50,000 cubic feet.
As each block is 1 cubic foot, this would require 50,000 blocks to make.

As the group size was 9, but one could not work, 8 people would be working on the obelisk.
In addition, as the day was nine schlibs long, but the workers rest for sixteen ponks (which equates to two schlibs), each worker would be building for seven schlibs a day.
They would each be able to lay 150 blocks per schlib, and per day this is 150 x 7 = 1050.

The whole group would therefore be able to lay 1050 x 8 = 8400 blocks a day.

Consequently, the obelisk would take a total of six days to complete, as in six days they could lay 8400 x 6 = 50,400 blocks (in five days this total would only be 42,000 blocks).

The Atlantian week has 5 days but only 4 of those would be spent working, so the work would be completed on the 2nd day of the 2nd week, which is Neptiminus.

Very similar thinking was used by Karim from Wilson's School:

1. The dimensions of the zin indicate that it contains 50,000 cubic feet of stone blocks.
2. The blocks are 1 cubic foot each, therefore, 50,000 blocks are required.
3. Each worker works 7 schlibs in a day (2 schlibs are devoted to rest).
4. Each worker lays 150 blocks per schlib, therefore each worker lays 1050 blocks per day.
5. There are 8 workers per day, therefore 8,400 blocks are laid per working day.
6. The 50,000th block, therefore, is laid on the sixth working day.
7. Since work does not take place on Daydoldrum, the sixth working day is Neptiminus.

Here is Mark's clearly laid out solution.

Edwin, Aufar, Hyeon and Dylan from B.S.M Muscat in Oman also found out that they finished building the Obelisk on Neptiminus. They added:

This puzzle was amazing!

Glad you enjoyed it. Well done to you all.