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Theoretical probability. Divisibility. Combinations. Experimental probability. Multiplication & division. Factors and multiples. Mathematical reasoning & proof. Modular arithmetic. Permutations. Probability.There are 29 results
Broad Topics > Numbers and the Number System > Modular arithmetic
Check Code Sensitivity
Age 14 to 16 Challenge Level:
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.
What Numbers Can We Make Now?
Age 11 to 14 Challenge Level:
Imagine we have four bags containing numbers from a sequence. What numbers can we make now?
Filling the Gaps
Age 14 to 16 Challenge Level:
Which numbers can we write as a sum of square numbers?
What Numbers Can We Make?
Age 11 to 14 Challenge Level:
Imagine we have four bags containing a large number of 1s, 4s, 7s and 10s. What numbers can we make?
Going Round in Circles
Age 11 to 14 Challenge Level:
Mathematicians are always looking for efficient methods for solving problems. How efficient can you be?
Elevenses
Age 11 to 14 Challenge Level:
How many pairs of numbers can you find that add up to a multiple of 11? Do you notice anything interesting about your results?
An Introduction to Modular Arithmetic
Age 14 to 16
An introduction to the notation and uses of modular arithmetic
Zeller's Birthday
Age 14 to 16 Challenge Level:
What day of the week were you born on? Do you know? Here's a way to find out.
Take Three from Five
Age 14 to 16 Challenge Level:
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?
Knapsack
Age 14 to 16 Challenge Level:
You have worked out a secret code with a friend. Every letter in the alphabet can be represented by a binary value.
Differences
Age 11 to 14 Challenge Level:
Can you guarantee that, for any three numbers you choose, the product of their differences will always be an even number?
Euler's Officers
Age 14 to 16 Challenge Level:
How many different ways can you arrange the officers in a square?
Novemberish
Age 14 to 16 Challenge Level:
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.
Transposition Fix
Age 14 to 16 Challenge Level:
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. . . .
Check Codes
Age 14 to 16 Challenge Level:
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. . . .
A One in Seven Chance
Age 11 to 14 Challenge Level:
What is the remainder when 2^{164}is divided by 7?
Odd Stones
Age 14 to 16 Challenge Level:
On a "move" a stone is removed from two of the circles and placed in the third circle. Here are five of the ways that 27 stones could be distributed.
Two Much
Age 11 to 14 Challenge Level:
Explain why the arithmetic sequence 1, 14, 27, 40, ... contains many terms of the form 222...2 where only the digit 2 appears.
Mod 3
Age 14 to 16 Challenge Level:
Prove that if a^2+b^2 is a multiple of 3 then both a and b are multiples of 3.
Obviously?
Age 14 to 18 Challenge Level:
Find the values of n for which 1^n + 8^n - 3^n - 6^n is divisible by 6.
Latin Squares
Age 11 to 18
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.
Days and Dates
Age 11 to 14 Challenge Level:
Investigate how you can work out what day of the week your birthday will be on next year, and the year after...
The Chinese Remainder Theorem
Age 14 to 18
In this article we shall consider how to solve problems such as "Find all integers that leave a remainder of 1 when divided by 2, 3, and 5."
How Much Can We Spend?
Age 11 to 14 Challenge Level:
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?
Where Can We Visit?
Age 11 to 14 Challenge Level:
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?
Grid Lockout
Age 14 to 16 Challenge Level:
What remainders do you get when square numbers are divided by 4?
Guesswork
Age 14 to 16 Challenge Level:
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.
The Best Card Trick?
Age 11 to 16 Challenge Level:
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?
NRICH is part of the family of activities in the Millennium Mathematics Project.