Modular arithmetic

  • Round and round and round
    problem

    Round and round and round

    Age
    11 to 14
    Challenge level
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    Where will the point stop after it has turned through 30 000 degrees? I took out my calculator and typed 30 000 ÷ 360. How did this help?

  • Elevenses
    problem

    Elevenses

    Age
    11 to 14
    Challenge level
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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?

  • How much can we spend?
    problem

    How much can we spend?

    Age
    11 to 14
    Challenge level
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    A country has decided to have just two different coins. Which totals can be made? Is there a largest total that cannot be made? How do you know?

  • Going round in circles
    problem

    Going round in circles

    Age
    11 to 14
    Challenge level
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    Mathematicians are always looking for efficient methods for solving problems. How efficient can you be?

  • What numbers can we make?
    problem

    What numbers can we make?

    Age
    11 to 14
    Challenge level
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    Imagine we have four bags containing a large number of 1s, 4s, 7s and 10s. What numbers can we make?

  • Where can we visit?
    problem

    Where can we visit?

    Age
    11 to 14
    Challenge level
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    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?

  • What numbers can we make now?
    problem

    What numbers can we make now?

    Age
    11 to 14
    Challenge level
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    Imagine we have four bags containing numbers from a sequence. What numbers can we make now?

  • Differences
    problem

    Differences

    Age
    11 to 14
    Challenge level
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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?

  • Two Much
    problem

    Two much

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

  • The Best Card Trick?
    problem

    The best card trick?

    Age
    11 to 16
    Challenge level
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    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?