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Play this game and see if you can figure out the computer's chosen number.

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Play the divisibility game to create numbers in which the first two digits make a number divisible by 2, the first three digits make a number divisible by 3...

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Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some. . . .

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Three people chose this as a favourite problem. It is the sort of problem that needs thinking time - but once the connection is made it gives access to many similar ideas.

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The number 8888...88M9999...99 is divisible by 7 and it starts with the digit 8 repeated 50 times and ends with the digit 9 repeated 50 times. What is the value of the digit M?

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Find the number which has 8 divisors, such that the product of the divisors is 331776.

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6! = 6 x 5 x 4 x 3 x 2 x 1. The highest power of 2 that divides exactly into 6! is 4 since (6!) / (2^4 ) = 45. What is the highest power of two that divides exactly into 100!?

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Find the highest power of 11 that will divide into 1000! exactly.

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Ben, Jack and Emma passed counters to each other and ended with the same number of counters. How many did they start with?

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A monkey with peaches, keeps a fraction of them each day, gives the rest away, and then eats one. How long can his peaches last?

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

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Take any four digit number. Move the first digit to the end and move the rest along. Now add your two numbers. Did you get a multiple of 11?

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Choose any 3 digits and make a 6 digit number by repeating the 3 digits in the same order (e.g. 594594). Explain why whatever digits you choose the number will always be divisible by 7, 11 and 13.

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Think of any three-digit number. Repeat the digits. The 6-digit number that you end up with is divisible by 91. Is this a coincidence?

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I'm thinking of a number. My number is both a multiple of 5 and a multiple of 6. What could my number be?

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The flow chart requires two numbers, M and N. Select several values for M and try to establish what the flow chart does.

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A three digit number abc is always divisible by 7 when 2a+3b+c is divisible by 7. Why?

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Can you order the digits from 1-3 to make a number which is divisible by 3 so when the last digit is removed it becomes a 2-figure number divisible by 2, and so on?

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

This article takes the reader through divisibility tests and how they work. An article to read with pencil and paper to hand.

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Look on the back of any modern book and you will find an ISBN code. Take this code and calculate this sum in the way shown. Can you see what the answers always have in common?

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The five digit number A679B, in base ten, is divisible by 72. What are the values of A and B?

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What is the remainder when 2^2002 is divided by 7? What happens with different powers of 2?

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What is the value of the digit A in the sum below: [3(230 + A)]^2 = 49280A

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A number N is divisible by 10, 90, 98 and 882 but it is NOT divisible by 50 or 270 or 686 or 1764. It is also known that N is a factor of 9261000. What is N?

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Is there an efficient way to work out how many factors a large number has?

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List any 3 numbers. It is always possible to find a subset of adjacent numbers that add up to a multiple of 3. Can you explain why and prove it?

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How many numbers less than 1000 are NOT divisible by either: a) 2 or 5; or b) 2, 5 or 7?

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Can you show that 1^99 + 2^99 + 3^99 + 4^99 + 5^99 is divisible by 5?

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

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In a square in which the houses are evenly spaced, numbers 3 and 10 are opposite each other. What is the smallest and what is the largest possible number of houses in the square?

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Powers of numbers behave in surprising ways. Take a look at some of these and try to explain why they are true.

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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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How many four digit square numbers are composed of even numerals? What four digit square numbers can be reversed and become the square of another number?

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When Charlie asked his grandmother how old she is, he didn't get a straightforward reply! Can you work out how old she is?

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This challenge encourages you to explore dividing a three-digit number by a single-digit number.

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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?

In this article for teachers, Bernard Bagnall describes how to find digital roots and suggests that they can be worth exploring when confronted by a sequence of numbers.

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