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Sets - Matrix DS

This topic has 3 expert replies and 4 member replies
kop Junior | Next Rank: 30 Posts Default Avatar
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Sets - Matrix DS

Post Wed Nov 20, 2013 12:43 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    A certain health club offers only yoga classes and spin classes. If all club members take at least one type of class, what percent of members take yoga classes?

    (1) 30% of members take both types of classes.

    (2) 40% of members take only spin classes.

    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ?? . If question was what percent of members take only Yoga classes??Then from both statements combined - 40% only Spin and 30% both spin and yoga - and remaning 30% only yoga ? is this right??

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    theCodeToGMAT Legendary Member
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    Post Wed Nov 20, 2013 1:30 am
    N1=Yoga
    N2=Spin

    To find: % of Yoga

    Weighted Average Formula
    N1/N2 = (M2-M)/(M-M1)

    Statement 1:
    M = 30
    M + M1 + M2 = 100
    M1 + M2 = 70
    We cannot guess M1
    INSUFFICIENT

    Statement 2:
    M2=40%
    M + M1 + M2 = 100
    M + M1 = 60
    SUFFICIENT

    Answer {B}

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    Last edited by theCodeToGMAT on Wed Nov 20, 2013 9:11 pm; edited 1 time in total

    Post Wed Nov 20, 2013 7:21 am
    kop wrote:
    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ??
    Yes, statement 2, alone, is sufficient.
    The correct answer is B

    I'll post a solution using the Double Matrix method shortly.

    Cheers,
    Brent

    _________________
    Brent Hanneson – Founder of GMATPrepNow.com
    Use our video course along with Beat The GMAT's free 60-Day Study Guide

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    Post Wed Nov 20, 2013 7:48 am
    kop wrote:
    A certain health club offers only yoga classes and spin classes. If all club members take at least one type of class, what percent of members take yoga classes?

    (1) 30% of members take both types of classes.
    (2) 40% of members take only spin classes.

    The Double Matrix Method can be used for most questions featuring a population in which each member has two characteristics associated with it.
    Here, we have a population of health club members, and the two characteristics are:
    - taking yoga or not taking yoga
    - taking spin classes or not taking spin classes

    So, we can set up our diagram as follows:

    IMPORTANT: Notice that I placed a zero in the bottom right corner because we are told that all club members take at least one type of class
    In other words, there are zero members who take neither yoga nor spin classes.

    Also notice that I made the total population of club members = 100. This is okay since the target question asks us to find a certain PERCENT.

    Target question: What percent of members take yoga classes?

    In other words, we want to find the sum of the top two boxes.


    Statement 1: 30% of members take both types of classes.
    So, we can place 30 of the 100 members in the top-left box.

    Does this provide enough information to find the sum of the top two boxes?
    No.
    Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

    Statement 2: 40% of members take only spin classes.
    So, we can place a 40 in the bottom left box.


    This means that a total of 40 of the 100 members do NOT take yoga.


    From this, we can conclude that the remaining 60 members DO take yoga.

    In other words, 60% of the members take yoga.
    Since we can answer the target question with certainty, statement 2 is SUFFICIENT

    Answer = B

    --------------------------------------------

    To learn more about the Double Matrix method, watch our free video: http://www.gmatprepnow.com/module/gmat-word-problems?id=919

    Then try these additional practice questions that can be solved using the Double Matrix Method:
    - http://www.beatthegmat.com/mba/2011/05/05/random-double-matrix-question-1
    - http://www.beatthegmat.com/mba/2011/05/09/random-double-matrix-question-2
    - http://www.beatthegmat.com/mba/2011/05/16/random-double-matrix-question-3
    - http://www.beatthegmat.com/ds-quest-t187706.html
    - http://www.beatthegmat.com/overlapping-sets-questions-t183320.html
    - http://www.beatthegmat.com/finance-majors-non-finance-majors-overlapping-set-question-t167425.html
    - http://www.beatthegmat.com/ds-french-japanese-t222297.html
    - http://www.beatthegmat.com/sets-t269449.html#692540
    - http://www.beatthegmat.com/in-costume-for-halloween-t269355.html#692116

    Cheers,
    Brent

    _________________
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    Use our video course along with Beat The GMAT's free 60-Day Study Guide

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    dddanny2006 Master | Next Rank: 500 Posts Default Avatar
    Joined
    12 Jan 2012
    Posted:
    209 messages
    Post Wed Nov 20, 2013 4:55 pm
    Why is this question any different then?

    How many members of Group G are less than 25 years of age?

    (1)Exactly 3/5 of the members of Group G are 25 years of age or older.
    (2)The 24 men in Group G constitute 30 percent of the groups membership.

    Here too Statement one could have been sufficient had we assumed a number for the people in the group.But the answer here is C.Hows that possible?This problem and the one thats in this forum contradict each other.



    Brent@GMATPrepNow wrote:
    kop wrote:
    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ??
    Yes, statement 2, alone, is sufficient.
    The correct answer is B

    I'll post a solution using the Double Matrix method shortly.

    Cheers,
    Brent

    Post Wed Nov 20, 2013 5:02 pm
    dddanny2006 wrote:
    Why is this question any different then?

    How many members of Group G are less than 25 years of age?

    (1)Exactly 3/5 of the members of Group G are 25 years of age or older.
    (2)The 24 men in Group G constitute 30 percent of the groups membership.

    Here too Statement one could have been sufficient had we assumed a number for the people in the group.But the answer here is C.Hows that possible?This problem and the one thats in this forum contradict each other.



    Brent@GMATPrepNow wrote:
    kop wrote:
    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ??
    Yes, statement 2, alone, is sufficient.
    The correct answer is B

    I'll post a solution using the Double Matrix method shortly.

    Cheers,
    Brent
    These two questions are similar in that they're questions involving overlapping sets, but the similarity ends there.

    In the original post, the target question asks us to determine what PERCENT of members take yoga classes.
    In the question you've posted here, the target question asks us to determine the NUMBER of members who are less than 25 years of age.

    Can you explain how these two questions contradict each other?

    Cheers,
    Brent

    _________________
    Brent Hanneson – Founder of GMATPrepNow.com
    Use our video course along with Beat The GMAT's free 60-Day Study Guide

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    Come see all of our free resources

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    dddanny2006 Master | Next Rank: 500 Posts Default Avatar
    Joined
    12 Jan 2012
    Posted:
    209 messages
    Post Wed Nov 20, 2013 5:05 pm
    Oh yes..I did overlook the percentages and numbers.But st 2 tells us that 60% may take only yoga or both yoga and spin,its not clearly mentioned.How can we be so sure?


    Please help me out with this question Brent.

    http://www.beatthegmat.com/brent-david-manhattan-tutor-rich-please-help-me-confused-t271929.html

    I dont understand the differences between method 1 and 2.

    Brent@GMATPrepNow wrote:
    dddanny2006 wrote:
    Why is this question any different then?

    How many members of Group G are less than 25 years of age?

    (1)Exactly 3/5 of the members of Group G are 25 years of age or older.
    (2)The 24 men in Group G constitute 30 percent of the groups membership.

    Here too Statement one could have been sufficient had we assumed a number for the people in the group.But the answer here is C.Hows that possible?This problem and the one thats in this forum contradict each other.



    Brent@GMATPrepNow wrote:
    kop wrote:
    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ??
    Yes, statement 2, alone, is sufficient.
    The correct answer is B

    I'll post a solution using the Double Matrix method shortly.

    Cheers,
    Brent
    These two questions are similar in that they're questions involving overlapping sets, but the similarity ends there.

    In the original post, the target question asks us to determine what PERCENT of members take yoga classes.
    In the question you've posted here, the target question asks us to determine the NUMBER of members who are less than 25 years of age.

    Can you explain how these two questions contradict each other?

    Cheers,
    Brent

    theCodeToGMAT Legendary Member
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    Posted:
    1556 messages
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    34 members
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    448 times
    Target GMAT Score:
    750
    GMAT Score:
    650
    Post Wed Nov 20, 2013 9:14 pm
    kop wrote:
    From ST2.. 60% remaning can take either only Yoga or yoga and spin classes. So to answer this question ST2 is enough ??right??
    Yes, because in this case people who participated in both "Yoga" & "Spin" will be counted... So {B}

    kop wrote:
    If question was what percent of members take only Yoga classes??Then from both statements combined - 40% only Spin and 30% both spin and yoga - and remaning 30% only yoga ? is this right??
    Yes, in this case we need "only" Yoga.. So answer will be {C}

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