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  1. #1
    Senior Member ptgatsby's Avatar
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    Default Random basic math question

    Tell me I'm not going crazy.

    I have this little bit of my math marked wrong;

    1965.75 = x + x(1+0.13/2)^1.5

    Can someone check what the answer to this should be? (I'd ask my tutor, except I want to make sure I'm being utterly blind. Excel and my calculator agree with me though, but I can't believe that their answer sheet is incorrect!)

  2. #2
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    Default

    1788.5615

    edit
    936.48723
    I forgot to add 1......

    1965,75 = x*[1+(1+0.13/2)'1.5]
    1965,75 = x*(1+1.065)'1.5
    1965,75= x*2.0990676
    x=936,48723

  3. #3
    filling some space UnitOfPopulation's Avatar
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    x = 938.392 (edit: 936.487 after typing the numbers correctly. I hate manually typing numbers. )

    I am not sure if I understood correctly. Here's what I did.

    1965.75 = x + x (1 + 0.13/2)^1.5
    <=>
    1965.75 = x + x (1.065)^1.5
    <=>
    1965.75 = x + 1.0990676x
    <=>
    1965.75 = x(1 + 1.0990676)
    <=>
    1965.75 = 2.0990676x
    <=>
    x= 1965.75/2.099067
    <=>
    x = 938.392
    (edit: last calculation 936.487). I don't know why it was wrong last time.

  4. #4
    Senior Member ptgatsby's Avatar
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    Wow, so far not one answer agreed with mine OR the official answer :P

    (Mine = 936.49, official = 1029.27)

  5. #5
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    Quote Originally Posted by ptgatsby View Post
    Wow, so far not one answer agreed with mine OR the official answer :P

    (Mine = 936.49, official = 1029.27)
    After revising, I agree with yours (look edit).
    I forgot to add 1 in the parenthesis

    edit again:
    And I agree with Santtu, until the x= 1965.75/2.099067
    but the division result???

  6. #6
    filling some space UnitOfPopulation's Avatar
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    Quote Originally Posted by ptgatsby View Post
    Wow, so far not one answer agreed with mine OR the official answer :P

    (Mine = 936.49, official = 1029.27)
    I made an error in the last part (probably wrote wrong numbers in the calculator). But assuming normal operator precedence, your and my (corrected) answer is correct. It may be that the official answer is incorrect. Have you preserved the correct use of parenthesis in the question?
    edit: I've had had to correct some IQ test makers who provided incorrect test answers to well-defined questions. Test makes aren't inerrand..

  7. #7
    Senior Member ptgatsby's Avatar
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    Quote Originally Posted by Santtu View Post
    I made an error in the last part (probably wrote wrong numbers in the calculator). But assuming normal operator precedence, your and my (corrected) answer is correct. It may be that the official answer is incorrect. Have you preserved the correct use of parenthesis in the question?
    edit: I've had had to correct some IQ test makers who provided incorrect test answers to well-defined questions. Test makes aren't inerrand..
    I'm going to question it now. This is just the end piece (the 8 lines before it were marked as correct, with my error being here), so maybe I'm not understanding the error location properly. Also, they may use variable answers (where the numbers change to prevent passing answers along) and there is some error in this particular one.

  8. #8
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    Quote Originally Posted by ptgatsby View Post
    I'm going to question it now. This is just the end piece (the 8 lines before it were marked as correct, with my error being here), so maybe I'm not understanding the error location properly. Also, they may use variable answers (where the numbers change to prevent passing answers along) and there is some error in this particular one.
    They are wrong if all the numbers are correct up to the last line (A number less than 2000 divided by a number larger than 2 will not result in more than 1000). This does not mean that you are correct, although in that case mistakes will be from typing in numbers wrong or something like that.

  9. #9
    Glowy Goopy Goodness The_Liquid_Laser's Avatar
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    The answer is x = 1965.75/(1+1.065^1.5) = 936.4872217

    I have spoken!
    My wife and I made a game to teach kids about nutrition. Please try our game and vote for us to win. (Voting period: July 14 - August 14)
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  10. #10
    Senior Member ptgatsby's Avatar
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    Quote Originally Posted by Zergling View Post
    They are wrong if all the numbers are correct up to the last line (A number less than 2000 divided by a number larger than 2 will not result in more than 1000). This does not mean that you are correct, although in that case mistakes will be from typing in numbers wrong or something like that.
    Right, so the teacher answered and gave me... uhhh... something that made no sense (copied from the answer sheet, no doubt). This is what I got back;

    1x + x(1+0.13/2)^1.5 =
    1x +x(1.065)^1.5 =
    1x + x(.9098621) = 1.909862x

    But of course (1.065)^1.5 cannot be less than 1... unless... it is actually;

    (1.065)^-1.5

    Which means I may have my PV/FV wrong... although it looks right to me!

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