### 4 May 2018: **Divisibility**

We can tell if a number is divisible by 3 by summing its digits and seeing if the result is divisible by 3!

The works because 10 = (3 * 3) + 1. So, each multiple of 10 that is added, is adding 1 modulo 3.

### 28 April 2018: **Puzzling Pennies**

*(From the Math Olympiad Contest Problems, Volume 1)* From a pile of 100 pennies, 100 nickels, and 100 dimes, select 21 coins that have a total value of exactly $1.00. In your selection you must also use at least one coin of each type. How many couns of each of the three types should be selected?

### 21 April 2018: **Coin Puzzle**

*(From the Math Olympiad Contest Problems, Volume 1)* I have 30 coins consisting of nickels and quarters. The total value of the coins is $4.10. How many of each kind do I have?

The tower below is made up of five horizontal layers of cubes with no gaps. How many individual cubes are in the tower?

### 7 April 2018: **Sorting Algorithms**

We did bubblesort (which took a lot of persistence by finished successfully!) and quicksort.

### 23 March 2018: **Fusing Dots**

Fusing Dots machine (from the Berkeley Math Circle): [PDF]

### 16 March 2018: **Smullyan’s Criminals**

Smullyan’s Criminals problems from the Berkeley Math Circle: [PDF]

### 9 March 2018: **Boolean Island**

We did some puzzles based on the Knights and Knaves from Raymond Smullyan’s *What is the Name of this Book?*: [PDF]

The last puzzle was to make up your own puzzle about the Boolies.

### 2 March 2018: **Water Rainbows**

We can make colored water with different densities by mixing in different amounts of sugar. Then, we can make a separated rainbow of different colored water in a straw!

### 23 February 2018: **Tree Adding**

### 9 February 2018: **Magic Squares**

Magic Squares (PPTX)

### 2 February 2018: **Magic Squares**

Two puzzles translated from “Culegere de Exercitii si Probleme pentru Concursul Gazeta Matematica Junior” clasa a II-a (Romanian Mathematics Junior Olympiad Problems)

Split the following numbers into two groups of equal sums: 2, 4, 5, 6, 8, 9.

Vladut wrote a sequence of numbers in increasing order, starting with the number 55. What is the twentieth digit written by Vladut?

### 26 January 2018: Flying Circuits!

### 8 December 2017: **Making Octahedrons with Stawbees**

### 1 December 2017: **Out with the Onion**

From Martin Gardner’s *My Best Mathematical and Logic Puzzles* (55): Move
two matches, to remake a class, but with the cherry outside.

**Dissection Dilemma** (57): divide a square into *five* parts, all
identical in size and shape.

### 24 November 2017: **Apples and Sticks**

Given three piles of 11, 7, and 6 stones, respectively. You can only move from one pile to another exactly as many stones are at the destination. In three moves, obtain three piles of 8 stones. (From

*The Moscow Puzzles*, by Boris A Kordemsky)First Dorina and Anya had a solution in four moves that starts with moving from the 11 stone pile to the 6 stone pile: (11,7,6), (5,7,12),(10,2,12),(8,4,12),(8,8,8). After a few tries, they figured out the three moves solution: (11,7,6), (4,14,6), (4,8,12), (8,8,8).

Five apples are in a basket. How do you divide them among five girls so that each girl gets and apple, but one apple remains in the basket. (From

*The Moscow Puzzles*, by Boris A Kordemsky) This was quite confusing, until David brought a prop. Then it seemed silly.Take 4 sticks of equal length and 4 more of equal length twice the first length. Use all 8 to make 3 squares of equal size. Itâ€™s acceptable to cross two long sticks. This was a pretty hard one. It took a while, but we finally did it.

### 17 November 2017: **Relatively Prime**

Everyone knows number lines (and number chains), but now its time for number rings!

If two numbers, *n* and *k* are relatively prime, we
will touch every number in the modulo number ring for *n* going
by multiples of *k*. Dorina picked 25 and 6, Anya picked 12 and
5, Charlotte picked 12 and 3 (not relatively prime, so doesn’t touch
every number), and Madelyn picked a number too big to finish.

(This is what we covered in cs2102: Discrete Mathematics the day before.)

### 10 November 2017: **Perimeters and Areas**

With 12 sticks, make a closed polygon with the largest area. With 12 sticks, make a closed polygon with the smallest integral area.

### 3 November 2017: **Pentagram**

With enough paper and colors we counted all 35 triangles!

### 28 October 2017: **Puzzling Pentagrams**

#### Moving Checkers

Place six checkers on a table in a row, alternating them black, white,
black, white, block white. Move the checkers so that all the white
one will end on the left, followed by the black ones. The checkers
must be moved in pairs, taking two adjacent at a time, without
disturbing the order, and sliding them to a vacant place. Only three
such moves are necessary. (From *The Moscow Puzzles*, by Boris A
Kordemsky)

This was a pretty successful problem. The girls enjoyed moving the stones around and find a solution.

#### Count!

How many different triangles are there in the pentagram figure? (From *The Moscow Puzzles*, by Boris A Kordemsky)

We were able to identify all the types of triangles, but we didn’t find a successful strategy for counting all 35 triangles, despite several persistent efforts from the girls.

### 21 October 2017: **Scrambled Box Tops**

Some more variations on the puzzle from 13 October.

### 13 October 2017: **Stick Puzzles**

Use 12 sticks to form polygons with integral areas. The entire length of the stick must be used with no overlapping, but more than one stick may be used for an edge. Assuming the sticks have unit length, what how many area units does your polygon have?

The solutions we had were: square with area 9 and rectangle with area 8. The challenge to get back to later: form a polygon of exactly four square units. (From

*My Best Mathematical and Logic Puzzels*by Martin Gardner, Puzzle 14, “The 12 Matches”.)Scrambled Box Tops

Imagine that you have three boxes, one containing two black marbles, one containing two white marbles, and the third, one black marble and one white marble. The boxes were labeled for their contents but someone has switched the labels so that every box in now incorrectly labeled. You are allowed to take one marble at a time out of any box, without looking inside, and by this process of sampling you are to determine the contents of all three boxes. What is the smallest number of drawing needed to do this? (From

*My Best Mathematical and Logic Puzzels*by Martin Gardner, Puzzle 5, “Scrambled Box Tops”.)This was a nice challenge, not too hard and kept all of us engaged. The temptation to pick in the box or get both marbles out was a bit too much. It worked best when three people were hiding the marbles one in each hand.

### 7 October 2017: **Electro-Dough**

Add some salt (and lemon juice) to dough, and it conducts! Let’s make some circuits.

### 23 September 2017: **Weight Puzzle**

There are twelve balls, eleven are the same, but one has a different weight from the others. How can you find the ball with the different weight using the minimum number of scale weighings?

### 16 September: **Mirror Symmetry**

What is the next image in the pattern?

### 9 September: **The Farmer, the Fox, the Goose, and the Grain**

The farmer has a fox (who will eat the goose), goose (who will eat the grain), and grain to bring back from the market in a boat that can only carry one at a time. How can the farmer get everyone safely to shore?

### 26 August 2017: **Eclipse**

On August 21 there was a total solar eclipse for parts of the US, and a near-total solar eclipse for Charlottesville. For the next math club, we wanted to understand what an Eclipse was, and why they are so rare.

The big purple ball represents the sun.

Radius of Sun | 432288 miles | Distance from Sun to Earth | 93 million miles = 215 sun radii |

Radius of the Earth | 3959 miles | Earth radii to Sun | 23,480 |

Radius of the Moon | 1079 miles | Distance from Earth to Moon | 239,800 miles = 60.5 earth radii |

So, to represent the Earth we need something roughly ^{1}⁄_{109} the radius
of the sun. For the big purple ball, which has radius about 10 inches,
we needed something with radius 0.01 inches - a small marble was the
closest readily available, but with its blue and green swirly color
made an excellent earth. The moon, ^{1}⁄_{3} the radius of the earth, was a
small piece of fuzz.

Scaling the size of the Sun down to the big purple ball, means the distance to the earth (normally 93 million miles) would scale down as 1 inch = 43228 miles. This mean the earth should be 179 feet away from the sun, and the moon would be 5.5 inches from the earth.

It actually worked! If you held the moon (fuzz) in just the right place, it would just occlude the sun.

We will be ready for the next eclipse in 2024!

### 13 April 2017: **Egg Coloring**

### 16 March 2017: **Polygons**

### 9 March 2017: **Shadowy Similar Triangles**

How tall is the tall, tall tree?Let's use our shadows to measure its shadow! Opps...it goes across the street. We'll have to try again closer to noon. |

### 2 March 2017: **Viral Fishy Stories**

(Inspired by *Tiddler: The Story-Telling Fish*)

How long does it take for a story to spread across the ocean? What if everyone tells the story to just one person? What if everyone tells the story to two new people? (We don’t have enough fish, but enough stuffies to get through 6 stages!)

(No pictures of actual math for this one, but lots of jumping in luggage!)

### 23 February 2017: **Crazy Calendars**

Calendars are puzzles - the last day of this month, first with the first day of the next month. There are different shapes of months because the number of days is either 30 or 31, and February is really odd.

Lucky for us, everyone’s birthday is between today and the end of June!

(Charlotte and Dorina are fighting over the June 29 square.)

Calendars (Excel spreadsheets, to print, use landscape mode and scale-to-fit): 31-day months, 30-day months

### 3 December 2016: **Hamiltonian Cycles!**

Find a path that starts from the castle and visits each character once, but cannot use any road more than once.

[PowerPoint]

### 24 September 2016: **Ruling Rulers**

### 28 August 2016: **Trying Triangles**

Can you make a triangle with any three sticks? Also, tracing the angles of any triangle to see they make a line!

### 21 August 2016: **Circles and Ellipses**

Drawing circles and ellipses with a string around thumbtacks on a board.

### 24 July 2016: **Water Ballistics**

### 17 July 2016: **Shadow Monsters**

Inspired by *The Gruffalo’s
Child*, we saw
who could make the biggest shadows.

### 9 April 2016: **Double Dice Probabilities**

### 26 March 2016: **Fibonacci Bunnies!**

Can’t have an Easter weekend meeting without lots of bunnies!

Mommy Bunny ⇒ Mommy Bunny and Baby Bunny

### 19 March 2016: **Set Theory: Unicorns, Pegasi, and Alicorns**

### 5 March 2016: **Transitivity!**

Balancing is *transitive*!

`A = B`

and `B = C`

means `A = C`

### 27 February 2016: **Commutativity!**

Mixing water colors is *commutative*! This means we end up with the
same color when we mix the same colors, regardless of the order in which
they are mixed. Mostly, though, we eventually end up with a murky, greenish-purple
color after enough colors are combined.

`Red + Blue = Purple = Blue + Red`