1. Joined
    08 Dec '03
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    3140
    29 Jul '04 18:17
    Using the digits 1 to 9, create two numbers which when multiplied together give you the highest number.

    For example, 12345678 * 9 = 111111102. Clearly there are higher products. What is the highest?

    -Ray.
  2. Zeist, Holland
    Joined
    11 Sep '03
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    19337
    30 Jul '04 17:47
    Could you elaborate? In what way is 111111102 the highest number?
  3. Joined
    08 Dec '03
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    3140
    30 Jul '04 18:191 edit
    Originally posted by piderman
    Could you elaborate? In what way is 111111102 the highest number?
    The original text does not state that 111111102 is the highest, but rather that there are higher products. The problem is to find the highest such product. 🙂

    -Ray.
  4. Zeist, Holland
    Joined
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    30 Jul '04 18:59
    OK. I understand now. 97531*8642=842862902. Try to beat that!
  5. Joined
    08 Dec '03
    Moves
    3140
    30 Jul '04 19:20
    Originally posted by piderman
    OK. I understand now. 97531*8642=842862902. Try to beat that!
    Believe it or not, there is a product that yields a slightly-larger result.

    -Ray.
  6. DonationAcolyte
    Now With Added BA
    Loughborough
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    3790
    30 Jul '04 19:33
    Originally posted by rgoudie
    Believe it or not, there is a product that yields a slightly-larger result.

    -Ray.
    9642*87531 = 843973902
  7. Joined
    08 Dec '03
    Moves
    3140
    30 Jul '04 19:59
    Originally posted by Acolyte
    9642*87531 = 843973902
    You got it.

    -Ray.
  8. Standard memberTheMaster37
    Kupikupopo!
    Out of my mind
    Joined
    25 Oct '02
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    20443
    07 Aug '04 20:49
    I see a trend here, give solution without the derivation...i wish to learn how one solves such a problem
  9. Joined
    08 Dec '03
    Moves
    3140
    24 Aug '04 21:27
    Originally posted by TheMaster37
    I see a trend here, give solution without the derivation...i wish to learn how one solves such a problem
    I apologize.

    In my future postings, I will include a solution if it is not supplied by the person answering.

    -Ray.
  10. Joined
    26 Apr '03
    Moves
    25837
    24 Aug '04 22:494 edits
    I'll try to explain:

    we have to make abcd * efghi as large as possible

    Multiplying this out and collecting together the powers of 10 this is equal to:
    10,000,000ae
    +1,000,000(be + af)
    +100,000(ag + bf + ce)
    +10,000(ah + bg + cf + de)
    +1,000(ai + bh + cg + df)
    +100(bi + ch + dg)
    +10(ci + dh)
    + di

    Considering the largest (most important) multiple of 10 we see that ae governs its multiplier value, therefore a and e must be 8 and 9, however a appears in more powers of 10 than e, so we will get the biggest answer if a=9, e=8. ie answer = 9bcd * 8fghi

    Next most important power of 10 is 1,000,000(8b + 9f) we can see that 7 and 6 must come in here and we get the largest multiplier by multiplying 8*6, 9*7 i.e b=6, f=7, now we have 96cd * 87ghi

    Next power of 10 is 100,000(9g + 42 + 8c), 5 and 4 must come in here and this will be largest if g=5, c=4, now we have 964d * 875hi

    Considering d and i, it is clear that these have the least impact, so must be where we allocate 1 and 2. i affects powers of 10 up to 1,000, whereas d affects them up to 10,000 so it is best if d=2, i=1: now we have 96c2 * 87gh1

    Next smallest numbers (3 and 4) look like they must go in c and h, c affects higher numbers than h, so c=4, h=3 will yield the largest product: 9642 * 87g31

    Filling in the missing 5: 9642 * 87531 = 843973902 is the largest product


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