Data Sufficiency- McGraw-Hill's GMAT 2008

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Data Sufficiency- McGraw-Hill's GMAT 2008

by hijazim » Tue Mar 17, 2009 9:58 pm
Q1.
Is r < 1?
(1) r > 0
(2) r^2 < 0

OA is B

My Answer is E:
stmt 1: r>0 => r>1 or 0<r<1 . Insufficient
stmt 2: r^2<0 => r is not a real number as all the square of any real number should be positive.

Q2.
If 5a = 9b = 15c, what is the value of a + b + c?
(1)3c &#8722; a = 5c &#8722; 3b
(2)6cb = 10a

OA is E

My Answer is C: Each statement alone is clearly insufficient.
stmt 1 x10: 30c-10a=50c-30b
Substitute stmt 2 in 1: 30c-6cb=50c-30b => 30b-20c=6cb
From the first given => c=(3/5)b ..... Substitute in the above equation

30b - 12b=6cb ..... Factor and reduce b
=>6c=18 => c = 3
Hence, a= 9 and b = 5 and a+b+c= 17 .

Therefore, the 2 statements together are sufficient.
What is wrong with my answer and why the OA is E

Please help in the above questions.
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by cramya » Tue Mar 17, 2009 10:19 pm
Q1.
Is r < 1?
(1) r > 0
(2) r^2 < 0


Is stmt II by any chance |r| <= 0 Then this would make r = 0 since it cant be less than 0 and therefore stmt II is sufficient

Still I feel it contradicts stmt I which says r>0 and in a real gmat question this wont happen. There wont be room for ambiguity. I cant think of any r when squared will be less than 0

GMAT only deals wiht real numbers if I am not mistaken so I feel its a little out of scope. Just my thought...

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by cramya » Tue Mar 17, 2009 10:25 pm
If 5a = 9b = 15c, what is the value of a + b + c?
(1)3c &#8722; a = 5c &#8722; 3b
(2)6cb = 10a

OA is E

My Answer is C: Each statement alone is clearly insufficient.
stmt 1 x10: 30c-10a=50c-30b
Substitute stmt 2 in 1: 30c-6cb=50c-30b => 30b-20c=6cb
From the first given => c=(3/5)b ..... Substitute in the above equation

30b - 12b=6cb ..... Factor and reduce b
=>6c=18 => c = 3
Hence, a= 9 and b = 5 and a+b+c= 17 .

Therefore, the 2 statements together are sufficient.
What is wrong with my answer and why the OA is E



Given:

Stmt II

5a = 9b = 15c

c=1/3a b = 5/9a

6cb = 10a

6*1/3*a*5/9*a = 10a

10/9 a^2 = 10a

a^2-9a = 0

a(a-9) = 0

a=0 or a=9

INSUFF


Stmt I
5a = 9b = 15c

3c &#8722; a = 5c &#8722; 3b

clealry not suff

Together:

Both conditions would be satisfied with a=0 or a=9

i.e if a=9 then b=5 c=3 a+b+c = 17

If a=0 b=0 c=0 a+b+c = 0

2 different values for a+b+c

Nice question

Hence E

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hijazim wrote:Q1.
Is r < 1?
(1) r > 0
(2) r^2 < 0
Is that really the question in the book? First, in real GMAT DS questions, the statements can't be contradictory, so even if Statement 2 read "r^2 <= 0", it would still be an unrealistic question (unless Statement 1 also read "r >= 0"). Second, all numbers on the GMAT are real numbers, so the inequality r^2 < 0 has no solutions whatsoever. That can't happen on a GMAT DS question - it must be possible for r to exist. Not a question worth spending any time on unless there's a typo in the original post above - is it possible that Statement 2 reads "r^2 < 1"?
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by hijazim » Wed Mar 18, 2009 11:00 pm
Yes Ian it is in the McGraw-Hill's GMAT 2008 preparation tests. I gues the book is not a very reliable source as I regret buying :). Thanks alot.