Skyscrapers

This is a quick post mainly for the benefit of my ‘Developing Mathematical Thinking’ (#math1070) students.

Introducing the puzzle

Skyscrapers are one of my favourite logic puzzles. They are a Japanese creation, introduced at the first World Puzzle Championship¹ in 1992.

Skyscrapers are a type of Latin Square puzzle. A Latin Square in an n × n grid filled with n different symbols, each occurring exactly once in each row and exactly once in each column. (Sudoku is another type of Latin Square puzzle).

In a Skyscraper puzzle the objective is to place a skyscraper in each square, with heights between 1 and n, so that no two skyscrapers in a row or column have the same height. The numbers (clues) on the outside of the grid tell us how many skyscrapers are visible from that position. (I like to imagine that I’m standing on that number and along the street that is the row/column.) Shorter skyscrapers aren’t visible behind taller ones.

We use logical deductions to solve the puzzle. For example, in the puzzle below, the clue ‘4’ tells us that the skyscrapers must appear in ascending height order in that row/column. Similarly, the clue ‘1’ tells us that the tallest skyscraper must be adjacent to the clue. That leads us to the partially-filled grid below. If you want to solve it yourself, the solution is at the bottom of this post. You can also play them online at Brain Bashers.

Hands-on skyscrapers

It is fairly easy to turn a skyscraper puzzle into a hands-on activity — just choose objects of different heights. Teachers often use linking cubes. You can also be more creative; at David Butler‘s One Hundred Factorial gathering at the University of Adelaide in May, we experimented with video cassettes (remember them?) and cups of different sizes.

A while back, I wanted to make several hands-on sets for 5 × 5 grids to use with groups of school students. They needed to be cheap, lightweight, compact and portable. So, I made paper cylinders that nestle inside each other. You can download and print the skyscraper cylinders. The tabs are meant to show where to overlap and tape. You can use them with these puzzles (print A3 size): Puzzle 1, Puzzle 2, Puzzle 3, Puzzle 4, Puzzle 5.

Skyscrapers in the classroom

My plan for MATH 1070 was curtailed by our short week (Week 3). I had planned the activity with these goals:

• Form visibly random groups with four students so that students could meet a few more classmates.
• Work collaboratively towards a common goal (and contrast this with the competitive nature of Prime Climb last week.)
• Practice claims and warrants as part of the focus on Maths Disputes: ‘I think <claim: this number goes here> because <warrant: my reason>.’

There are a variety of reasons to use skyscrapers in the classroom; you might like to read these posts by teachers: Mary Bourassa, Mark Chubb, Sarah Carter. (Update 22 January 2021: Erick Lee has created an awesome Desmos version of Skyscraper Puzzles.)

Any activity introduced into the classroom should be intentional. You might like to think about these dot points. Mark has a fuller list in his blog post.

• If giving these puzzles to individual students is different than to groups of students.
• If a physical model is different than a pen-and-paper version.
• If you’ll use it as part of a lesson or as a ‘time filler’.
• What you’ll do if students give up easily.

If you give them a go, let me know what you think!

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[1] Source: www.gmpuzzles.com/blog/skyscrapers-rules-and-info