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  1. #1
    fatabass is offline Senior Member
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    Default How would you solve this? Java coding for multiplication.

    *** ( Some number with 3 digits )
    ** ( Some number with 2 digits )
    x
    -------------
    ****
    ****
    -----------
    *****

    where all the asterisks are prime digits either: 2,3,5 or 7

    Ideas, approaches, or even code mayde?

  2. #2
    JosAH's Avatar
    JosAH is offline Moderator
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    Default Re: How would you solve this? Java coding for multiplication.

    It cannot be solved because the righrmost digit in the fourth row has to be a zero.

    kind regards,

    Jos
    The only person who got everything done by Friday was Robinson Crusoe.

  3. #3
    fatabass is offline Senior Member
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    Default Re: How would you solve this? Java coding for multiplication.

    What makes you think that?

    I am sure it is a "can be solved" question.

  4. #4
    pbrockway2 is offline Moderator
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    Default Re: How would you solve this? Java coding for multiplication.

    Quote Originally Posted by JosAH View Post
    righrmost
    @OP: Can you post again using code tags, because it makes a difference. Put [code] at the start of the calculation and [/code] at the end. That way the formatting will be preserved and we can all see what is supposed to line up with what.

  5. #5
    fatabass is offline Senior Member
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    Default Re: How would you solve this? Java coding for multiplication.

    How would you solve this? Java coding for multiplication.-puzzle_prime.jpg

  6. #6
    JosAH's Avatar
    JosAH is offline Moderator
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    Default Re: How would you solve this? Java coding for multiplication.

    The last digit of the top two numbers have to be a 3 and a 5 (...3x..5 = ....5 (or ..5x.3 = ....5 is the only possibility), so the last digit of the result is 5 as well. Maybe consistent reasoning like this can help you to solve this puzzle. If there are too many alternatives I'd take the lazy approach (brute force)

    kind regards,

    Jos
    The only person who got everything done by Friday was Robinson Crusoe.

  7. #7
    fatabass is offline Senior Member
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    Default Re: How would you solve this? Java coding for multiplication.

    I am trying to solve it with Java :)
    Thanks though... :)

    I have this idea of looping for all numbers in the first 2 rows and multiplying them and to check if they are all prime numbers(2,3,5,7)..

    But can there be a better approach ?

  8. #8
    pbrockway2 is offline Moderator
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    Default Re: How would you solve this? Java coding for multiplication.

    Totally slow answer adding nothing removed.

    @OP: That's more or less what I would do.
    Last edited by pbrockway2; 12-23-2011 at 10:17 PM.

  9. #9
    JosAH's Avatar
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    Default Re: How would you solve this? Java coding for multiplication.

    There are four different digits (2, 3, 5 and 7) so you should count in base 4. There are 4x4x4 possible first numbers and 4x4 possible second numbers. You can try them all after mapping 0 --> 2, 1 --> 3, 2 --> 5 and 3 --> 7 and see which numbers have the correct result.

    kind regards,

    Jos
    The only person who got everything done by Friday was Robinson Crusoe.

  10. #10
    fatabass is offline Senior Member
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    Default Re: How would you solve this? Java coding for multiplication.

    Thank you

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