# Resources tagged with: Combinatorics

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There are 55 NRICH Mathematical resources connected to Combinatorics, you may find related items under Decision Mathematics and Combinatorics.

Broad Topics > Decision Mathematics and Combinatorics > Combinatorics

### In a Box

##### Age 14 to 16Challenge Level

Chris and Jo put two red and four blue ribbons in a box. They each pick a ribbon from the box without looking. Jo wins if the two ribbons are the same colour. Is the game fair?

### Snooker Frames

##### Age 16 to 18Challenge Level

It is believed that weaker snooker players have a better chance of winning matches over eleven frames (i.e. first to win 6 frames) than they do over fifteen frames. Is this true?

### Binomial Coefficients

##### Age 14 to 18

An introduction to the binomial coefficient, and exploration of some of the formulae it satisfies.

### Ip?

##### Age 16 to 18Challenge Level

Seventh challenge cipher

### Jluuis or Even Asutguus?

##### Age 16 to 18Challenge Level

Sixth challenge cipher

### Up a Semitone?

##### Age 16 to 18Challenge Level

Fifth challenge cipher

### Semicircle

##### Age 16 to 18Challenge Level

Fourth challenge cipher

### Vital?

##### Age 16 to 18Challenge Level

Third challenge cipher

### A Fine Thing?

##### Age 16 to 18Challenge Level

Second challenge cipher

### Stage 5 Cipher Challenge

##### Age 16 to 18Challenge Level

Can you crack these very difficult challenge ciphers? How might you systematise the cracking of unknown ciphers?

### An Introduction to Computer Programming and Mathematics

##### Age 16 to 18

This article explains the concepts involved in scientific mathematical computing. It will be very useful and interesting to anyone interested in computer programming or mathematics.

### Molecular Sequencer

##### Age 14 to 18Challenge Level

Investigate the molecular masses in this sequence of molecules and deduce which molecule has been analysed in the mass spectrometer.

### Symmetric Tangles

##### Age 14 to 16

The tangles created by the twists and turns of the Conway rope trick are surprisingly symmetrical. Here's why!

### Tangles

##### Age 11 to 16

A personal investigation of Conway's Rational Tangles. What were the interesting questions that needed to be asked, and where did they lead?

### Cube Net

##### Age 16 to 18Challenge Level

How many tours visit each vertex of a cube once and only once? How many return to the starting point?

### Lost in Space

##### Age 14 to 16Challenge Level

How many ways are there to count 1 - 2 - 3 in the array of triangular numbers? What happens with larger arrays? Can you predict for any size array?

### Bell Ringing

##### Age 11 to 14Challenge Level

Suppose you are a bellringer. Can you find the changes so that, starting and ending with a round, all the 24 possible permutations are rung once each and only once?

### Ordered Sums

##### Age 14 to 16Challenge Level

Let a(n) be the number of ways of expressing the integer n as an ordered sum of 1's and 2's. Let b(n) be the number of ways of expressing n as an ordered sum of integers greater than 1. (i) Calculate. . . .

### Factorial Fun

##### Age 16 to 18Challenge Level

How many divisors does factorial n (n!) have?

### Postage

##### Age 14 to 16Challenge Level

The country Sixtania prints postage stamps with only three values 6 lucres, 10 lucres and 15 lucres (where the currency is in lucres).Which values cannot be made up with combinations of these postage. . . .

### Counting Binary Ops

##### Age 14 to 16Challenge Level

How many ways can the terms in an ordered list be combined by repeating a single binary operation. Show that for 4 terms there are 5 cases and find the number of cases for 5 terms and 6 terms.

### Magic W Wrap Up

##### Age 16 to 18Challenge Level

Prove that you cannot form a Magic W with a total of 12 or less or with a with a total of 18 or more.

### Ways of Summing Odd Numbers

##### Age 11 to 14

Sanjay Joshi, age 17, The Perse Boys School, Cambridge followed up the Madrass College class 2YP article with more thoughts on the problem of the number of ways of expressing an integer as the sum. . . .

### An Investigation Based on Score

##### Age 11 to 14

Class 2YP from Madras College was inspired by the problem in NRICH to work out in how many ways the number 1999 could be expressed as the sum of 3 odd numbers, and this is their solution.

### The Eternity Puzzle

##### Age 16 to 18

A big prize was offered for solving The Eternity Puzzle, a jigsaw with no picture and every piece is the same on both sides. The finished result forms a regular dodecagon (12 sided polygon).

### Transitivity

##### Age 16 to 18

Suppose A always beats B and B always beats C, then would you expect A to beat C? Not always! What seems obvious is not always true. Results always need to be proved in mathematics.

##### Age 14 to 18

Some puzzles requiring no knowledge of knot theory, just a careful inspection of the patterns. A glimpse of the classification of knots, prime knots, crossing numbers and knot arithmetic.

### Master Minding

##### Age 11 to 14Challenge Level

Your partner chooses two beads and places them side by side behind a screen. What is the minimum number of guesses you would need to be sure of guessing the two beads and their positions?

##### Age 11 to 14Challenge Level

Is it possible to use all 28 dominoes arranging them in squares of four? What patterns can you see in the solution(s)?

### Plate Spotting

##### Age 7 to 11Challenge Level

I was in my car when I noticed a line of four cars on the lane next to me with number plates starting and ending with J, K, L and M. What order were they in?

### Deep Roots

##### Age 14 to 16Challenge Level

Find integer solutions to: $\sqrt{a+b\sqrt{x}} + \sqrt{c+d.\sqrt{x}}=1$

### Paving Paths

##### Age 11 to 14Challenge Level

How many different ways can I lay 10 paving slabs, each 2 foot by 1 foot, to make a path 2 foot wide and 10 foot long from my back door into my garden, without cutting any of the paving slabs?

### Euromaths

##### Age 11 to 14Challenge Level

How many ways can you write the word EUROMATHS by starting at the top left hand corner and taking the next letter by stepping one step down or one step to the right in a 5x5 array?

##### Age 14 to 16Challenge Level

A walk is made up of diagonal steps from left to right, starting at the origin and ending on the x-axis. How many paths are there for 4 steps, for 6 steps, for 8 steps?

### One Basket or Group Photo

##### Age 7 to 18Challenge Level

Libby Jared helped to set up NRICH and this is one of her favourite problems. It's a problem suitable for a wide age range and best tackled practically.

### How Many Dice?

##### Age 11 to 14Challenge Level

A standard die has the numbers 1, 2 and 3 are opposite 6, 5 and 4 respectively so that opposite faces add to 7? If you make standard dice by writing 1, 2, 3, 4, 5, 6 on blank cubes you will find. . . .

### Flagging

##### Age 11 to 14Challenge Level

How many tricolour flags are possible with 5 available colours such that two adjacent stripes must NOT be the same colour. What about 256 colours?

### Greetings

##### Age 11 to 14Challenge Level

From a group of any 4 students in a class of 30, each has exchanged Christmas cards with the other three. Show that some students have exchanged cards with all the other students in the class. How. . . .

### Snowman

##### Age 14 to 16Challenge Level

All the words in the Snowman language consist of exactly seven letters formed from the letters {s, no, wm, an). How many words are there in the Snowman language?

### Cube Paths

##### Age 11 to 14Challenge Level

Given a 2 by 2 by 2 skeletal cube with one route `down' the cube. How many routes are there from A to B?

### Penta Colour

##### Age 14 to 16Challenge Level

In how many different ways can I colour the five edges of a pentagon red, blue and green so that no two adjacent edges are the same colour?

### Scratch Cards

##### Age 14 to 16Challenge Level

To win on a scratch card you have to uncover three numbers that add up to more than fifteen. What is the probability of winning a prize?

### Tri-colour

##### Age 11 to 14Challenge Level

Six points are arranged in space so that no three are collinear. How many line segments can be formed by joining the points in pairs?

### Painting Cubes

##### Age 11 to 14Challenge Level

Imagine you have six different colours of paint. You paint a cube using a different colour for each of the six faces. How many different cubes can be painted using the same set of six colours?

### W Mates

##### Age 16 to 18Challenge Level

Show there are exactly 12 magic labellings of the Magic W using the numbers 1 to 9. Prove that for every labelling with a magic total T there is a corresponding labelling with a magic total 30-T.

### Knight Defeated

##### Age 14 to 16Challenge Level

The knight's move on a chess board is 2 steps in one direction and one step in the other direction. Prove that a knight cannot visit every square on the board once and only (a tour) on a 2 by n board. . . .

### Magic W

##### Age 14 to 16Challenge Level

Find all the ways of placing the numbers 1 to 9 on a W shape, with 3 numbers on each leg, so that each set of 3 numbers has the same total.

### Olympic Magic

##### Age 14 to 16Challenge Level

in how many ways can you place the numbers 1, 2, 3 … 9 in the nine regions of the Olympic Emblem (5 overlapping circles) so that the amount in each ring is the same?

### Plum Tree

##### Age 14 to 18Challenge Level

Label this plum tree graph to make it totally magic!

### Magic Caterpillars

##### Age 14 to 18Challenge Level

Label the joints and legs of these graph theory caterpillars so that the vertex sums are all equal.