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Is there a simpler way to solve this question?

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shibsriz@gmail.com GMAT Destroyer! Default Avatar
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Is there a simpler way to solve this question? Post Tue Jun 11, 2013 3:54 am
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  • Lap #[LAPCOUNT] ([LAPTIME])
    166. What is the value of x + y in the figure above?
    (1) w = 95
    (2) z = 125
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    Post Tue Jun 11, 2013 5:16 am
    shibsriz@gmail.com wrote:


    What is the value of x + y in the figure above?
    (1) w = 95
    (2) z = 125
    Target question: What is the value of x + y?

    Statement 1: w = 95

    Important: For geometry DS questions, we are typically checking to see whether the statements "lock" a particular angle or length into having just one value. This concept is discussed in much greater detail in our free video: http://www.gmatprepnow.com/module/gmat-data-sufficiency?id=1103

    If w = 95, then the angle inside the quadrilateral must be 85.

    So, those 2 angles (95 and 85) are "locked." In other words, the 2 lines that create those two angles are locked in place to create the 95- and 85-degree angles.
    However, line1 is not locked into place, so we can still move it, which means we can freely alter the size of angle y.
    As such, the value of x + y will vary.
    Since we cannot answer the target question with certainty, statement 1 is SUFFICIENT

    Statement 2: z = 125
    If z = 125, then the angle inside the quadrilateral must be 55.

    Since line2 is not locked into place, we can still move it, which means we can freely alter the size of angle x.
    As such, the value of x + y will vary.
    Since we cannot answer the target question with certainty, statement 2 is SUFFICIENT

    Statements 1 and 2 combined:
    We now have the following:

    Since all angles in a quadrilateral add to 360 degrees, we know that 85 + 55 + j + k = 360
    If we solve for j + k, we get: j + k = 220

    Also notice that, since angles x and k are on a line, it must be true that x + k = 180.
    Similarly, it must be true that y + j = 180
    If we combine both of these equations, we get: x + y + j + k = 360
    Since we already know that j + k = 220, we can replace j + k with 220, to get:
    x + y + 220 = 360
    This means x + y = 140
    Since we can not answer the target question with certainty, the combined statements are SUFFICIENT

    Answer = C

    Cheers,
    Brent

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    GaneshMalkar Rising GMAT Star Default Avatar
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    Post Tue Jun 11, 2013 9:16 am
    One more way to do this :-

    Let the interior angles be a,b,c,d in the clockwise direction.

    Then a+b+c+d = 360 ...(a closed quadrilateral adds angle to 360)

    So, (180-x) + (180-y) + (180-w) + (180-z) = 360

    720 - (x+y+w+z) = 360
    x+y+w+z = 360

    x + y = 360 - (w + z) ... We need to know the values of w and z to get x + y

    (a) w = 95
    Not Sufficient since we need value of z.

    (b) z = 125
    Not Sufficient since we need value of w.

    (a) and (b)

    Sufficient.

    Answer C

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    Post Fri Apr 01, 2016 10:32 am
    Dear All,

    Request you to please guide me how can we assume it to be quadrilateral when it is not that the two lines are parallel . As we have learnt in the basic that we should not assume seeing the figure then can we infer that the two lines are parallel.

    Post Fri Apr 01, 2016 10:37 am
    Rohit_Prakash88@yahoo.in wrote:
    Dear All,

    Request you to please guide me how can we assume it to be quadrilateral when it is not that the two lines are parallel . As we have learnt in the basic that we should not assume seeing the figure then can we infer that the two lines are parallel.
    There need not be any parallel sides for a shape to be a quadrilateral.
    A quadrilateral is any 4-sided polygon.

    Cheers,
    Brent

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    Post Sat Apr 02, 2016 12:48 pm
    GaneshMalkar wrote:
    One more way to do this :-

    Let the interior angles be a,b,c,d in the clockwise direction.

    Then a+b+c+d = 360 ...(a closed quadrilateral adds angle to 360)

    So, (180-x) + (180-y) + (180-w) + (180-z) = 360

    720 - (x+y+w+z) = 360
    x+y+w+z = 360

    x + y = 360 - (w + z) ... We need to know the values of w and z to get x + y

    (a) w = 95
    Not Sufficient since we need value of z.

    (b) z = 125
    Not Sufficient since we need value of w.

    (a) and (b)

    Sufficient.

    Answer C
    Ganesh's solution is my recommended approach on this question!

    On DS geometry questions, a lot of students get into trouble if they begin by plugging the statement information into the figure right away. If there is a given figure, it's much better to make any and all inferences you can before looking at the statements. Otherwise, you might think that you need statement information that was already inferable from the figure.

    Here's an example of a DS geometry problem that student often get wrong if they don't make inferences first:
    http://www.beatthegmat.com/area-of-the-circumscribed-circle-t276376.html#719010

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