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few princeton review DS questions from 2nd ed math workout

This topic has 4 member replies
coachy Newbie | Next Rank: 10 Posts Default Avatar
Joined
11 Feb 2007
Posted:
3 messages

few princeton review DS questions from 2nd ed math workout

Post Mon Feb 19, 2007 2:25 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    On Page 222 they have thus DS question
    Is it true that X < Y?
    1 5x < 5y
    2 zx < zy

    I thought the answer should be D. They say it is A.
    Explaination is that in second statement Z could be +ve or -ve.

    I chose D because we are already given zx < zy. Isn't it true that we have to assume that statement 2 is true.


    Similarly on page 223

    the question is
    If X and Y are integers, does x = y?
    1) xy = y2 (2 here is square)
    2) x2 = y2 (2 here is square)

    I thought the answer should be again D but the book says it is C.

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    jayhawk2001 Community Manager
    Joined
    28 Jan 2007
    Posted:
    789 messages
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    29 times
    Post Mon Feb 19, 2007 4:02 pm
    [quote="coachy"]On Page 222 they have thus DS question
    Is it true that X < Y?
    1 5x < 5y
    2 zx < zy

    I thought the answer should be D. They say it is A.
    Explaination is that in second statement Z could be +ve or -ve.

    I chose D because we are already given zx < zy. Isn't it true that we have to assume that statement 2 is true.


    [/quote]

    [size=18]x < y holds true from 5x < 5y for all values of x and y. So, (1) is
    sufficient.

    zx < zy does not necessarily mean x < y, as we don't know whether
    z is positive or negative. So, (2) is insufficient.

    Hence A.

    [/size]

    [quote="coachy"]
    Similarly on page 223

    the question is
    If X and Y are integers, does x = y?
    1) xy = y2 (2 here is square)
    2) x2 = y2 (2 here is square)

    I thought the answer should be again D but the book says it is C.[/quote]

    Can you please explain why this should be D ?

    (1) is sufficient as you can safely cancel out y from both sides
    leading to x = y

    I'd say (2) is insufficient as x^2 = y^2 can be expressed as
    (x+y)(x-y)=0 which means x = y or x=-y.

    kandelaki Senior | Next Rank: 100 Posts Default Avatar
    Joined
    05 Feb 2007
    Posted:
    37 messages
    Post Mon Feb 19, 2007 10:48 pm
    Is it true that X < Y?
    1 5x < 5y
    2 zx < zy


    well if we assume that z=0 then x is not < than y , they both will be 0
    that is why only first choice gives sufficient explanation.......

    coachy Newbie | Next Rank: 10 Posts Default Avatar
    Joined
    11 Feb 2007
    Posted:
    3 messages
    Post Tue Feb 20, 2007 7:53 am
    if we assume z = 0 even the 2nd statement would not be true...

    zx < zy

    tells us that z cannot be 0.

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    kandelaki Senior | Next Rank: 100 Posts Default Avatar
    Joined
    05 Feb 2007
    Posted:
    37 messages
    Post Tue Feb 20, 2007 10:46 pm
    you're right.
    I guess it just about z being positve or negative, nothing to do with 0.
    thanks

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