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Algebra

This topic has 4 expert replies and 1 member reply
John fran kennedi Junior | Next Rank: 30 Posts
Joined
02 Feb 2016
Posted:
26 messages
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Algebra

Post Wed Mar 09, 2016 10:25 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])



    Hello guys, I have just practiced the GMAT, as we can see on this image , I get confused to solve this. I have seen the answer explanation , but I don't understand well. I don't know whether because I don't get the question well or not.
    In the answer explanation ( OG 2016) page 245, it states
    =10,000(0.0012)-100(0.0012)
    =12,12 - 0,12

    I don't know why it's 12,12, not 12.

    I do need your help to make me understand. Thanks in advance

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    Post Wed Mar 09, 2016 10:34 am
    Quote:
    A bar over a sequence of digits in a decimal indicates that the sequence repeats indefinitely. What is the value of (10^4 - 10^2) (0.0012)?

    Note: There is a bar over "12" in 0.0012.

    (A) 0
    (B) 0.12 (with the bar over the 12)
    (C) 1.2
    (D) 10
    (E) 12
    (10⁴ - 10²)(0.00121212...) = 10⁴(0.00121212...) - 10²(0.00121212...)
    = 12.121212... - 0.121212...
    = 12 (since the decimal parts, in blue, are identical, they cancel out]

    Answer: E

    Cheers,
    Brent

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    Post Wed Mar 09, 2016 10:35 am
    Quote:
    A bar over a sequence of digits in a decimal indicates that the sequence repeats indefinitely. What is the value of (10^4 - 10^2) (0.0012)?

    Note: There is a bar over "12" in 0.0012.

    (A) 0
    (B) 0.12 (with the bar over the 12)
    (C) 1.2
    (D) 10
    (E) 12
    We can also apply some number sense here.

    (10⁴ - 10²)(0.00121212...) = (10,000 - 100)(0.00121212...)
    = 9900(0.00121212...)
    = (a bit less than 10,000)(0.00121212...)

    Well, (10,000)(0.00121212...) = 12.121212
    So, (a bit less than 10,000)(0.00121212...) = a bit less than 12.121212...

    Check the answer choices:
    A) 0
    B) 0.121212...
    C) 1.2
    D) 10
    E) 12

    E is the best match

    Cheers,
    Brent

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    John fran kennedi Junior | Next Rank: 30 Posts
    Joined
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    Posted:
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    1 times
    Post Wed Mar 09, 2016 10:47 am
    Wow, I really do understand now. Thank you so much

    GMAT/MBA Expert

    Post Thu Mar 17, 2016 9:26 pm
    Another idea here: since (xy/99) = .xyxyxyxy..., we can say that

    .0012121212... =

    .12121212... * (1/100) =

    12/99 * 1/100 =

    12/9900

    Now we have

    (10⁴ - 10²) * 12/9900 =

    9900 * 12/9900 =

    12

    and we're set!

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    Post Mon Oct 09, 2017 7:33 am
    A common mistake that students make on a problem like this is to subtract the exponents. But of course we cannot subtract the exponents when we're subtracting terms with the same base.

    More on this here: http://www.beatthegmat.com/fdp-problem-og-ps-174-t288679.html#763953

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