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virtually all books/gmat courses teach the power of "picking number" strategies...

however, at least from what i understand, very rarely teach when NOT to you it. it can be an hazardous methodology sometimes....

the more obvious cases are when 1,2,3 and even 4 examples that "goes right" doesn't tell us that something will always (for any number)"go right".
this usually happens in DS questions, but not always...

the other, less obvious case when you SHOULD avoid "picking numbers", is where this method creates lots of "work", that is, computations and arihmetics. especially if these involve fractions, negative numbers and exponents.

most of us will eventually will do some mistakes in calculations.

true.... we can be careful and avoid those mistake (a wise move)... but, we can altogether avoid the computation itself.

like in this question. in order to find if a multiplication is positive or negative we should just ask if each component is positive or negative... not try numbers to see whats happenning there....

i claimed before and i still claim, succeeding gmat quant is not about knowing enough math techniques for solving problems, but rathr - using as few math skills as possible to solve them.

like in this question. in order to find if a multiplication is positive or negative we should just ask if each component is positive or negative

hobbit, how would you have reasoned this w/o picking at least one set of #s (i.e. [-1^2 - ((-1)(0)) & -1^2 - ((-1)(1))])

I know in order to be safe picking #s I should've tried fractions too. In that way, this methodology is flawed. Anyone, please explain how you approached/would approach this prob.

Statement 2 says that x < y, so y is either 0 or positive.

Now you have POSITIVE NUMBER - (NEGATIVE NUMBER)0

OR

POSITIVE NUMBER - (NEGATIVE NUMBER)(NEGATIVE NUMBER)

kpcronin,

Did you mean POSITIVE NUMBER - (NEGATIVE NUMBER)(POSITIVE NUMBER) instead of POSITIVE NUMBER - (NEGATIVE NUMBER)(NEGATIVE NUMBER)?
I like your explanation. I just wanna make sure I fully understand it.

succeeding gmat quant is not about knowing enough math techniques for solving problems, but rathr - using as few math skills as possible to solve them.

Great comment. I couldn't agree more. For each question there may be multiple ways to solve it. Some of them may need to use more advanced math concepts and skills. But most GMAT questions can be solved using the most basic math skills.

ggarr wrote:

Quote:

like in this question. in order to find if a multiplication is positive or negative we should just ask if each component is positive or negative

hobbit, how would you have reasoned this w/o picking at least one set of #s (i.e. [-1^2 - ((-1)(0)) & -1^2 - ((-1)(1))])

As hobbit said, you shouldn't use picking numbers for this question.
x(x-y) can be positive in two scenarios:
a. x>0, x-y>0 (meaning x>y)
b. x<0, x-y<0 (meaning x<y)

Since 1 and 2 combined gives us the second senario, we can determine that x(x-y) is indeed positive. Therefore C. _________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

Is x(x-y) positive?
x<0
x<y
----------------------------

Lesson: Pick numbers only when you can't do it with variables or in your head..

consider1)

x is -ve , y can be -ve or zero or +ve
now, whats the stem ? y is zero or +ve -> stem is +ve.
but y is -ve then if y is < x ..stem is -ve
if y is >x , then stem is +ve
( here look at 2) and [b]you can see answer is C.[/b]

TF, cant conclude from 1) Insuffi.

Take 2) x<y

say x is +ve, -> y is also +ve and >x _> stem is -ve

say x is -ve -> y can be +ve or -ve or zero
based on y value, stem can be -ve or +ve

cant conclude ..insuffi..

Take both:

stem is +ve

therefore answer is C.

Lesson: Pick numbers only when you can't do it with variables or in your head..

virtually all books/gmat courses teach the power of "picking number" strategies...

however, at least from what i understand, very rarely teach when NOT to you it. it can be an hazardous methodology sometimes....

the more obvious cases are when 1,2,3 and even 4 examples that "goes right" doesn't tell us that something will always (for any number)"go right". this usually happens in DS questions, but not always...

the other, less obvious case when you SHOULD avoid "picking numbers", is where this method creates lots of "work", that is, computations and arihmetics. especially if these involve fractions, negative numbers and exponents.

most of us will eventually will do some mistakes in calculations.

true.... we can be careful and avoid those mistake (a wise move)... but, we can altogether avoid the computation itself.

like in this question. in order to find if a multiplication is positive or negative we should just ask if each component is positive or negative... not try numbers to see whats happenning there....

i claimed before and i still claim, succeeding gmat quant is not about knowing enough math techniques for solving problems, but rathr - using as few math skills as possible to solve them.