Questions from Princeton Review Adaptive test 4

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1. If 'x' and 'y' are positive integers (4^x)(1/3)^y <1?

(1) y=2x
(2) y=4

2. Is a^2 - a >= 0

(1) 7>=a>=1
(2) 3>=a>=0

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by microke » Mon Aug 04, 2008 10:21 pm
I think for qn 1,the eqn can be simplified as

is 4^x3^-y<1? from statement 1 , the eqn becomes is4^x/ 3^(x^2) <1? you can try substituting any positive integers for x. The outcome is always less than 1. Hence statement 1 is sufficient.

No information of x in statement 2. So Ans is A.

For qn no 2, statement 1 fails for the condition a=1 where 0>0 is undefined and also statement 2 fails for the condition a=0. Hence answer can be E.

BTW, what is the OA?

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by pranavc » Wed Aug 06, 2008 6:54 am
The OA for question 1 is C (I arrived at A too and I cannot figure out why C is the answer)

The OA for question 2 is A (don't know why)

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by jsl » Wed Aug 06, 2008 7:13 am
pranavc wrote:The OA for question 2 is A (don't know why)
Hiya pranavc,

Hope I can help you with the second ne.... The equation basically asks is a^2 greater or equal to a?

in statement 1, you only get positive integers and when you square a postive integer, it always gets bigger. Statement 1 is sufficient

in statement 2, this includes positive fractions and when you square a positive fraction, it gets smaller. However, for numbers 2 and 3, the number increases again. Therefore statement 2 is insufficient

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by aspire750 » Wed Aug 06, 2008 9:29 am
pranavc wrote:The OA for question 1 is C (I arrived at A too and I cannot figure out why C is the answer)

The OA for question 2 is A (don't know why)
The reason is:

Q1. Statement1. y=2x, suppose y=1, in this case x=1/2
plugging these values in the question stem can be
2*0.33=0.66 or -2*0.33=-0.66 so, no definite answer because sqrt4=+-2. Therefore, 1 alone is insufficient

Statement2. y=4. It is insufficient to answer as no information about x

Now, 1+2, when y=4, x=2 which is sufficient to supply definite answer. Hence C

Ques2. The question is Is a(a-1)>=0
Statement1. 7>=a>=1
If a=1.5, it is >0 ;for all other values in the range it satisfies ,hence Sufficient

Statement2.3>=a>=0
when a=0.5, the value of a(a-1) will be negative. However, for other positive values between 1 &3, it will hold good. So, Insufficient

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by cornell2005 » Wed Aug 06, 2008 11:15 am
I don't get question 1 though... apparently the question stated that both x and y are postive integers. The explanation above said suppose x = 1/2. Am i missing something? Thanks.

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by microke » Wed Aug 06, 2008 7:54 pm
Thanks everyone for correcting me....In Qn1, I still have doubt as like cornel2005. for qn no 2, i misread the question ignoring equal to zero, considering only the condition as greater than zero....

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by Ian Stewart » Thu Aug 07, 2008 4:35 am
1. If 'x' and 'y' are positive integers (4^x)(1/3)^y <1?

(1) y=2x
(2) y=4


If y = 2x, then, using the basic exponent rules,

(4^x)(1/3)^y = (4^x)(1/3)^(2x) = (4^x)/(3^(2x)) = (4^x)/(9^x) = (4/9)^x

If x is a positive integer, (4/9)^x clearly must be less than 1.

The answer is A, not C, so either the OA is incorrect (which happens often enough), or the question has been mistranscribed.
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 microke » Thu Aug 07, 2008 10:51 pm
Hmm. If OA is incorrect, then it is difficult for candidate who follow reputed materials, in trust that the information given will be correct. Can the originator post the explanation of the OA, so that candidates will be aware of such important mistakes?