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Am I missing something?? (no calculator on this?)

This topic has 1 expert reply and 6 member replies
tzink Senior | Next Rank: 100 Posts Default Avatar
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Am I missing something?? (no calculator on this?)

Post Thu Jul 10, 2008 9:12 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    The question is from the OG

    If the units digit of integer n is greater than 2, what is the units digit of n?

    1) The units digit of n is the same as the units digit of n^2

    2) The units digit of n is the same as the units digit of n^3

    The correct answer is E, but what I'm more worried about is the guide's explanation of how to arrive at the answer:

    "To solve this problem, it is necessary to find a digit that is the same as the units digit of its square. For example, both 43 squared (1,849) and 303 squared (91,809) [why of course! i know all my squares up to a million by heart!] have a units digit of 9, which is different from the units digit of 43 and 303. However, 25 squared (625) and 385 squared (148,225) both have a units digit of 5, and 16 and 225 both have a units digit of 6 and their squares (256 and 51,076) do, too. There is no other information to choose between 5 or 6, so (1) is not sufficient."

    Even worse, when explaining why the second statement is insufficient, they do the same thing with cubes! How on earth are we expected to do this without a calculator? and in two minutes?? unreal!

    Are there shorter methods to this? just guess and move on?

    cheers

    Thanked by: sanju09
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    louvre Junior | Next Rank: 30 Posts Default Avatar
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    Post Fri Jul 11, 2008 7:07 am
    From 1. Only those numbers with 5 and 0 in their unit's place satisfy the given condition. But we can't pin on one of the two possibilities.

    From 2. It filters to the same possibilities of 5 and 0.

    So none of the 2 clues is sufficient.

    cubicle_bound_misfit Master | Next Rank: 500 Posts
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    Post Fri Jul 11, 2008 7:37 am
    Leran the cyclicity for finding unit digits.

    2 has a cyclicity of 4

    i.e 2 sqare unit dig :4
    2 cube unit digit : 8
    2 to the power four unit digit : 6

    so, cyclicity 2-4-8-6 then repeat from 2^5

    similarly 3

    3-9-7-1

    for 4
    4-6-

    for 5
    5-
    for 6
    6

    for 7
    7 -9-3-1
    for 8
    8-4-2-6
    for 9
    9-1

    So you can easily see for both the conditions 5 and 6 will have cube and squares unit digit same.

    Hope this will help.

    regards,

    _________________
    Cubicle Bound Misfit

    Thanked by: tzink
    tzink Senior | Next Rank: 100 Posts Default Avatar
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    Post Fri Jul 11, 2008 7:58 am
    yeah, that makes it much more manageable..
    why wouldn't the OG tell me that?

    cubicle_bound_misfit Master | Next Rank: 500 Posts
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    Post Fri Jul 11, 2008 8:07 am
    There are more things in heaven and earth, TZINK
    Than are taught by OG.

    That's why let's beat the s_ out of GMAT.



    Regards,

    _________________
    Cubicle Bound Misfit

    rajman41 Newbie | Next Rank: 10 Posts Default Avatar
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    Post Wed Feb 10, 2010 10:43 pm
    What's tat " E " for answers mean Error or 5=E( alphabet , a=1, b=2......).
    5 satisfies both the requirement to be a solid answer. Why didn't they choose #5?

    GMAT/MBA Expert

    sanju09 GMAT Instructor
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    Post Wed Feb 10, 2010 11:33 pm
    tzink wrote:
    The question is from the OG

    If the units digit of integer n is greater than 2, what is the units digit of n?

    1) The units digit of n is the same as the units digit of n^2

    2) The units digit of n is the same as the units digit of n^3

    The correct answer is E, but what I'm more worried about is the guide's explanation of how to arrive at the answer:

    "To solve this problem, it is necessary to find a digit that is the same as the units digit of its square. For example, both 43 squared (1,849) and 303 squared (91,809) [why of course! i know all my squares up to a million by heart!] have a units digit of 9, which is different from the units digit of 43 and 303. However, 25 squared (625) and 385 squared (148,225) both have a units digit of 5, and 16 and 225 both have a units digit of 6 and their squares (256 and 51,076) do, too. There is no other information to choose between 5 or 6, so (1) is not sufficient."

    Even worse, when explaining why the second statement is insufficient, they do the same thing with cubes! How on earth are we expected to do this without a calculator? and in two minutes?? unreal!

    Are there shorter methods to this? just guess and move on?

    cheers
    Sorry tzink, I was about to click “Quote” icon to write my reply but it slipped and clicked otherwise.

    Besides, (1) is possible with unit’s digit 5 or 6 only. Insufficient

    (2) This for a second time happens with unit’s digit 4, 5, 6, or 9. Insufficient

    Taken together in turn leaves us with 5 or 6 to choose from. Insufficient

    E

    _________________
    The mind is everything. What you think you become. –Lord Buddha



    Sanjeev K Saxena
    Quantitative Instructor
    The Princeton Review - Manya Abroad
    Lucknow-226001

    www.manyagroup.com

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    bynddrvn Senior | Next Rank: 100 Posts Default Avatar
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    Post Sat Sep 20, 2014 7:05 am
    The creators of the test assume you will be able to figure out shortcuts.

    Although, I must confess I do not understand why the test makers insist on sticking to solving mathematical problems with a stylus and tablet when the rest of the civilized world uses calculators or spreadsheets. Heck, I know welders and plumbers who use spreadsheets and most MBA programs require you bring a laptop to class.

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