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Broad Topics > Numbers and the Number System > Modulus arithmetic

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Going Round in Circles

Stage: 3 Challenge Level: Challenge Level:1

Mathematicians are always looking for efficient methods for solving problems. How efficient can you be?

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How Much Can We Spend?

Stage: 3 Challenge Level: Challenge Level:1

A country has decided to have just two different coins, 3z and 5z coins. Which totals can be made? Is there a largest total that cannot be made? How do you know?

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Elevenses

Stage: 3 Challenge Level: Challenge Level:1

How many pairs of numbers can you find that add up to a multiple of 11? Do you notice anything interesting about your results?

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Guesswork

Stage: 4 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Ask a friend to choose a number between 1 and 63. By identifying which of the six cards contains the number they are thinking of it is easy to tell them what the number is.

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Take Three from Five

Stage: 3 and 4 Challenge Level: Challenge Level:2 Challenge Level:2

Caroline and James pick sets of five numbers. Charlie chooses three of them that add together to make a multiple of three. Can they stop him?

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Where Can We Visit?

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

Charlie and Abi put a counter on 42. They wondered if they could visit all the other numbers on their 1-100 board, moving the counter using just these two operations: x2 and -5. What do you think?

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Differences

Stage: 3 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Can you guarantee that, for any three numbers you choose, the product of their differences will always be an even number?

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Prime AP

Stage: 4 Challenge Level: Challenge Level:1

Show that if three prime numbers, all greater than 3, form an arithmetic progression then the common difference is divisible by 6. What if one of the terms is 3?

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Days and Dates

Stage: 4 Challenge Level: Challenge Level:1

Investigate how you can work out what day of the week your birthday will be on next year, and the year after...

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What Numbers Can We Make Now?

Stage: 3 and 4 Challenge Level: Challenge Level:2 Challenge Level:2

Imagine we have four bags containing numbers from a sequence. What numbers can we make now?

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Filling the Gaps

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

Which numbers can we write as a sum of square numbers?

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What Numbers Can We Make?

Stage: 3 Challenge Level: Challenge Level:1

Imagine we have four bags containing a large number of 1s, 4s, 7s and 10s. What numbers can we make?

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Weekly Challenge 8: Sixinit

Stage: 5 Short Challenge Level: Challenge Level:1

Choose any whole number n, cube it and add 11n. Is the answer always divisible by 6? If so why?

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Weekly Challenge 41: Happy Birthday

Stage: 5 Challenge Level: Challenge Level:2 Challenge Level:2

A weekly challenge concerning the interpretation of an algorithm to determine the day on which you were born.

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Zeller's Birthday

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

What day of the week were you born on? Do you know? Here's a way to find out.

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Elevens

Stage: 5 Challenge Level: Challenge Level:1

Add powers of 3 and powers of 7 and get multiples of 11.

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Shuffles

Stage: 5 Challenge Level: Challenge Level:1

An environment for exploring the properties of small groups.

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The Knapsack Problem and Public Key Cryptography

Stage: 5

An example of a simple Public Key code, called the Knapsack Code is described in this article, alongside some information on its origins. A knowledge of modular arithmetic is useful.

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Knapsack

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

You have worked out a secret code with a friend. Every letter in the alphabet can be represented by a binary value.

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The Public Key

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Find 180 to the power 59 (mod 391) to crack the code. To find the secret number with a calculator we work with small numbers like 59 and 391 but very big numbers are used in the real world for this.

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Double Time

Stage: 5 Challenge Level: Challenge Level:2 Challenge Level:2

Crack this code which depends on taking pairs of letters and using two simultaneous relations and modulus arithmetic to encode the message.

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Readme

Stage: 5 Challenge Level: Challenge Level:1

Decipher a simple code based on the rule C=7P+17 (mod 26) where C is the code for the letter P from the alphabet. Rearrange the formula and use the inverse to decipher automatically.

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Modular Fractions

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

We only need 7 numbers for modulus (or clock) arithmetic mod 7 including working with fractions. Explore how to divide numbers and write fractions in modulus arithemtic.

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Transposition Fix

Stage: 4 Challenge Level: Challenge Level:1

Suppose an operator types a US Bank check code into a machine and transposes two adjacent digits will the machine pick up every error of this type? Does the same apply to ISBN numbers; will a machine. . . .

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Check Code Sensitivity

Stage: 4 Challenge Level: Challenge Level:1

You are given the method used for assigning certain check codes and you have to find out if an error in a single digit can be identified.

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Check Codes

Stage: 4 Challenge Level: Challenge Level:1

Details are given of how check codes are constructed (using modulus arithmetic for passports, bank accounts, credit cards, ISBN book numbers, and so on. A list of codes is given and you have to check. . . .

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Obviously?

Stage: 4 and 5 Challenge Level: Challenge Level:1

Find the values of n for which 1^n + 8^n - 3^n - 6^n is divisible by 6.

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Pythagoras Mod 5

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Prove that for every right angled triangle which has sides with integer lengths: (1) the area of the triangle is even and (2) the length of one of the sides is divisible by 5.

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Rational Round

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Show that there are infinitely many rational points on the unit circle and no rational points on the circle x^2+y^2=3.

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Dirisibly Yours

Stage: 5 Challenge Level: Challenge Level:1

Find and explain a short and neat proof that 5^(2n+1) + 11^(2n+1) + 17^(2n+1) is divisible by 33 for every non negative integer n.

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A One in Seven Chance

Stage: 3 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

What is the remainder when 2^{164}is divided by 7?

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The Best Card Trick?

Stage: 3 and 4 Challenge Level: Challenge Level:1

Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?

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Latin Squares

Stage: 3, 4 and 5

A Latin square of order n is an array of n symbols in which each symbol occurs exactly once in each row and exactly once in each column.

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Modulus Arithmetic and a Solution to Dirisibly Yours

Stage: 5

Peter Zimmerman from Mill Hill County High School in Barnet, London gives a neat proof that: 5^(2n+1) + 11^(2n+1) + 17^(2n+1) is divisible by 33 for every non negative integer n.

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More Sums of Squares

Stage: 5

Tom writes about expressing numbers as the sums of three squares.

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Modulus Arithmetic and a Solution to Differences

Stage: 5

Peter Zimmerman, a Year 13 student at Mill Hill County High School in Barnet, London wrote this account of modulus arithmetic.

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Small Groups

Stage: 5

Learn about the rules for a group and the different groups of 4 elements by doing some simple puzzles.

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Clock Squares

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

Square numbers can be represented on the seven-clock (representing these numbers modulo 7). This works like the days of the week.

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Two Much

Stage: 3 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Explain why the arithmetic sequence 1, 14, 27, 40, ... contains many terms of the form 222...2 where only the digit 2 appears.

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Grid Lockout

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

What remainders do you get when square numbers are divided by 4?

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Modular Knights

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Try to move the knight to visit each square once and return to the starting point on this unusual chessboard.

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Euler's Officers

Stage: 4 Challenge Level: Challenge Level:1

How many different solutions can you find to this problem? Arrange 25 officers, each having one of five different ranks a, b, c, d and e, and belonging to one of five different regiments p, q, r, s. . . .

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Remainder Hunt

Stage: 5 Challenge Level: Challenge Level:2 Challenge Level:2

What are the possible remainders when the 100-th power of an integer is divided by 125?

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Novemberish

Stage: 4 Challenge Level: Challenge Level:1

a) A four digit number (in base 10) aabb is a perfect square. Discuss ways of systematically finding this number. (b) Prove that 11^{10}-1 is divisible by 100.

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Mod 7

Stage: 5 Challenge Level: Challenge Level:1

Find the remainder when 3^{2001} is divided by 7.

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Mod 3

Stage: 4 Challenge Level: Challenge Level:1

Prove that if a^2+b^2 is a multiple of 3 then both a and b are multiples of 3.

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Old Nuts

Stage: 5 Challenge Level: Challenge Level:1

In turn 4 people throw away three nuts from a pile and hide a quarter of the remainder finally leaving a multiple of 4 nuts. How many nuts were at the start?

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More Mods

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

What is the units digit for the number 123^(456) ?

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Purr-fection

Stage: 5 Challenge Level: Challenge Level:1

What is the smallest perfect square that ends with the four digits 9009?

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It Must Be 2000

Stage: 2 Challenge Level: Challenge Level:2 Challenge Level:2

Here are many ideas for you to investigate - all linked with the number 2000.