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1089

Posted by Chris on July 17, 2010 – 7:18 pm

A friend of mine found this:

Take a 3 digit number abc where a≠c. Reverse it. Subtract the smaller from the larger. Add the result to the reverse of the result. You get 1089. Prove it.

I really only posted it because of the previous puzzle (Reversing Numbers)


This post is under “Mathemagic” and has 23 respond so far.
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23 Responds so far- Add one»

  1. 1. Nathan Said:

    111 is a three digit number that will not satisfy the requirements. 111-111+111=1111089

  2. 2. Chris Said:

    Thanks Nathan. I’ll fixed it. But you should have done 111-111 = 0, then 0 + 0 = 0.

  3. 3. Jhowmar Said:

    hmm ok so i didn’t think there was a way to do it and i’m pretty sure i was right

    first off the middle number has to be 9

    any middle number will = 9 during the subtraction

    to get the 8 makes it so the middle number had to be 9 it can’t be 8 because there is no way to make 19 in the 1’s colom

    so this means that the reversed number + the result must = 9

    well you can always get the 89 but you can’t get the 10 in

    actual problem using abc as the large number when reversed

    abc – cba = xyz
    xyz + zyx = 1089

  4. 4. Jhowmar Said:

    sorry forgot to post where i found the info from
    http://www.mathsisfun.com/1089.html

    also let me clarify this
    to get the 8(in the 10’s colom) makes it so the middle number had to be 9 it(the middle number) can’t be 8 because there is no way to make 19 in the 1’s colom

    not sure if it was needed or not

  5. 5. Anonymous Said:

    Take a 3 digit number abc where a≠c.
    Reverse it.
    Subtract the larger from the smaller.
    Add the result to the reverse of the result.
    You get 1089.
    Prove it.

    S=100*a-10*b+1*c-100*c-10*b-1*a=99*a-99*c
    S=99*a-99*c=99*(a-c)

    S=100*(a-c)-(a-c)
    Consider 100*(a-c). If we subtract a quantity of (a-c) from it
    a-c is less than or equal to 9 thus first digit is (a-c)-1.
    a-c is less than or equal to 9 thus second digit must be 9
    last digit is 10-(a-c) or 10-a+c

    S’=reverse of S
    first digit is 10-(a-c)
    second digit is 9
    last digit is (a-c)-1.

    S+S’=100*((a-c-1)+(10-a+c))+10*(9+9)+1*((10-a+c)+(a-c-1))
    S+S’=100*((a-c-1)+(10-a+c))+10*(9+9)+1*((10-a+c)+(a-c-1))
    S+S’=100*(9)+10*(9+9)+1*(9)
    S+S’=900+180+9=1089

    However the assumptions brake down if a-c=1 as the middle digit does not exist
    a-c=1 S=99 S’=99 S+S’=99+99=198 Counterexample!! e.g 645-546=99 99+99=198

    So You only get 1089 if a-c is greater than 1.

    Cam

  6. 6. Anonymous Said:

    I should briefly state that abc in the calculations is chosen to be the larger of the two numbers.

    Cam

  7. 7. Anonymous Said:

    Some clarification on this part
    “a-c is less than or equal to 9 thus first digit is (a-c)-1.
    a-c is less than or equal to 9 thus second digit must be 9
    last digit is 10-(a-c) or 10-a+c”

    a-c is actually less than or equal to 8

    Since a-c is less than or equal to 8:
    a-c is a number between 1 and 99 thus first digit is (a-c)-1.
    a-c is a number between 1 and 9 thus second digit must be 9
    last digit is 10-(a-c) or 10-a+c

    Cam

  8. 8. slavy Said:

    Well Cam, it depends how you reverse the difference. If you always reverse it as a 3-digit number (i.e., 99 is actually 099 and its reverse is 990) then the answer is still correct even for a-c=1. The rest is OK, but I just want to point out that (at least for me) is easier to work not with 3 variables a, b, and c, but with only one, if you make the observation (you have still done it but you didn’t use the full benefit of it) that there is a one-to-one correspondence between numbers of the type a9(9-a) (these are the digits of a tr-digit number) and the absolute values of the difference of an arbitrary 3-digit number and its reverse (in the case that the first and the last digit doesn’t coincide!). Here, a is (a-c-1) in your notation and it can be between 0 and 8. Now it is straightforward that the sum a9(9-a)+(9-a)9a=1089 (again, if a=0, we reverse the number in a stranger way).

  9. 9. Chris Said:

    Thanks for rescuing that slavy. I did it simply by doing the calculations using positional notation with a,b,c instead of explicit numbers, including the borrowing and carrying back. Using “,” to separate the digits.

    NB a > c
    a,b,c -
    c,b,a
    —–
    a-(c+1),(10+b)-(b+1),10-(a-c) =
    (a-c)-1,9,10-(a-c) then add to it’s reverse
    10-(a-c),9,(a-c)-1 +
    ——————
    9+1,8,9
    = 1089

    @slavy, is this significant to the other problem? (Only kidding, I think).

  10. 10. Anonymous Said:

    Hey slavvy,

    Thanks for the input.

    I realized when doing that if you did 990+099=1089, but it seemed pretty hokey to me to do it that way.

    If I were to ask a random person to reverse the digits of 12 I would expect 21. If they replied 21,000 ,with the justification that they could write 12 with 3 leading 0s, I would be surprised. One could justify it, but it would be a stretch.

    I’m not a big fan of reducing the 3 variables down to 1 for this problem following reasons:
    -have to justify/proove the 1 to 1 correspondance of the digits (not a big deal, but I’m lazy)
    -easier to explain/justify solution when it is seen all the initial variables cancel out (which they almost immediately do). Less appearance of hocus pocus, and it seems more elegant (but this is, of course, subjective).

    I think that if there was more steps involved in cancelling out the variables, or if the steps were complicated, I might have gone in that direction.

    It just goes to show that, oddly enough, math is a discipline which has lots of room for individual style and creativity. It’s interesting to see how different people tackle a problem in different ways to get to the same result.

    Cam

  11. 11. ino Said:

    @nathan, 111 can’t be an example since it should be a≠c, 111 makes a=c, that is, 1=1.

  12. 12. Chris Said:

    @ino, I added the a≠c because of and after Nathan’s post.

  13. 13. slavy Said:

    Hi, Chris! So far I don’t see connection between this problem and the previous one, but I haven’t thought much about it either. If I found something in the future, I will let you know :)

    @Cam – I am not criticizing your solution :) I agree with most of your arguments (apart from proving the one-to-one correspondence, because you have basically done it and you could have just set c-a-1 to be a new variable A) and I just rewrote your solution in a less formalized way. And yes, in general reversing with a 0 in front is kind of magic, but since here we talk about 3-digit numbers from the beginning it has some justification.

  14. 14. Chris Said:

    @slavy. I only really thought that it was an amusing coincidence.

  15. 15. John24 Said:

    Poorly worded question.

    Take a 3 digit number abc where a≠c. Reverse it. Subtract the larger from the smaller. Add the result to the reverse of the result. You get 1089. Prove it.

    The above states that if my number is 321 to reverse it and subtract the larger from the smaller. 123 – 321 = -198 and add the reverse of it.

    It should read subtract smaller from largest. 321 – 123 = 198.
    Add the result to the reverse of the result. 198 + 891 = 1089.

    Proving it would take less than 30 minutes.

  16. 16. Chris Said:

    @John24. Well spotted, my bad. I’ve fixed it for posterity. It took me about 5 minutes to solve, doing the borrows and carries did my nut in.

  17. 17. John24 Said:

    This is far from a proof, but is curious nevertheless.

    Note: ABC – CBA / 99 is always an integer when A≠C.

    99 990
    198 891
    297 792
    396 693
    495 594
    594 495
    693 396
    792 297
    891 198
    990 99

    Notice all combination = 1089 when added. Also, 1089 / 99 = 11.

    Suppose: ABC – CBA = DEF if FED is the reverse of DEF then FED + DEF must = 1089.

  18. 18. John24 Said:

    Using similar logic to the original question…

    All 5 digit numbers have a total of 109890. Coincidentally ABCDE – EDCBA is divisible by 99. Also, 109890 / 99 = 1110.

    All 7 digit numbers have a total of 10998900. Coincidentally ABCDEFG – GFEDCBA is divisible by 99. Also, 10998900 / 99 = 111100

  19. 19. Chris Said:

    I clicked on a random link and found this page. I note that John24 says that (ABC-CBA)/99 is an integer. He’s right.

    ABC-CBA = 100A + 10B + C – 100C -10B – A = 100(A-C) -(A-C) = 99(A-C). If A = C then obviously get 0.

  20. 20. Lewis Said:

    100(c-a)=d
    10(b-b)=e
    (a-c)=f
    100(f+d)=900 ALWAYS
    10(e+e)=180 ALWAYS (this gives it the 1000 and the 80)
    (f+d)=9 ALWAYS
    in total that gives you 1089

    Lewis: 14
    Solevd in: 30 minutes

  21. 21. Chris Said:

    Hi Lewis. You have made a bit of a mess of it. Possibly that only happened when you were writing it up, and possibly because you introduced the factors of 10 and 100 too early.

    A glaring error is that you have 10(b-b) = e and that gives e = 0.
    But you say 10(e+e) = 180 even though it must be 0.

    I suspect that you had meant something more like as follows:

    Let c:b:a – a:b:c = d:e:f – I’m using “:” to avoid confusion with multiplication.
    Because c > a, when we subtract, the units part requires that we borrow from
    the tens i.e. we calculate f = (10+a)-c. But then to do the tens subtraction, we
    must borrow from the hundreds to find e = (10+b) – (b+1) = 9. Finally for the hundreds we calculate d = c – (a+1) and there’s no need to borrow.

    So d+f = (c – (a+1)) + ((10 + a) – c) = 9 and e = 9.

    Then d:e:f + f:e:d = (100d + 10e + f) + (100f + 10e + d) = 101(d+f) + 20e
    = 909 + 180 = 1089.

  22. 22. Chamroeun Said:

    Hi, you can check this link: http://www.khmath.com/magic-number-1089/

  23. 23. korte Said:

    I Can Find the Number you Thought! The magic number!!!

    Try it…!!!

    https://youtu.be/2YcfuJXrm7A

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