GMATPrep DS Obstacles

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myfish: Senior | Next Rank: 100 Posts; Posts: 67; Joined: Mon May 03, 2010 12:03 pm; Thanked: 3 times

by myfish » Sat Mar 03, 2012 1:36 pm

I have the right answers from GMATPrep but no idea how they got there, even after 10 min

Please take a look at the screenshots and take a crack at it. Thanks a lot!!!!!

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Source: Beat The GMAT — Data Sufficiency | Sat Mar 03, 2012 1:36 pm

kul512: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Thu Sep 29, 2011 5:15 amFollowed by: 1 member; Thanked: 7 times; GMAT Score:740

by kul512 » Sat Mar 03, 2012 6:46 pm

i deleted this post...
I dont want to confuse others with wrong solutions

Last edited by kul512 on Sat Mar 03, 2012 10:54 pm, edited 1 time in total.

Sometimes there is very fine line between right and wrong: perspective.

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kul512: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Thu Sep 29, 2011 5:15 amFollowed by: 1 member; Thanked: 7 times; GMAT Score:740

by kul512 » Sat Mar 03, 2012 6:56 pm

i deleted this post as i dont want to confuse others with wrong solution.

Last edited by kul512 on Sat Mar 03, 2012 10:55 pm, edited 1 time in total.

Sometimes there is very fine line between right and wrong: perspective.

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kul512: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Thu Sep 29, 2011 5:15 amFollowed by: 1 member; Thanked: 7 times; GMAT Score:740

by kul512 » Sat Mar 03, 2012 7:06 pm

i deleted this post as i dont want to confuse others with wrong solution.

Last edited by kul512 on Sat Mar 03, 2012 10:56 pm, edited 1 time in total.

Sometimes there is very fine line between right and wrong: perspective.

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myfish: Senior | Next Rank: 100 Posts; Posts: 67; Joined: Mon May 03, 2010 12:03 pm; Thanked: 3 times

by myfish » Sat Mar 03, 2012 9:25 pm

Careful. GMATPrep says the answer is A. I assume this first step is troublesome

"as r2+2 is a positive number so question reduces to - ((r2+2)/p)<r"

Thanks for the effort anyways. I am sure its something simple I am overseeing, just can't pinpoint it.

[quote="kul512"][size=18][b]Question3----[/b][/size]

Question asks whether-(1/p)<(r/(r2+2))
as r2+2 is a positive number so question reduces to -
((r2+2)/p)<r
((r2+2-pr)/p)<0

A. putting p=r
(r2+2-r2)/p<0
(2/p)<0

but as we don't know the sign of p we cant say anything.
NOT SUFFICIENT

B. r>0
only sign of r only cant say anything about the inequality as sign of P is also required.
NOT SUFFICIENT

Now combining the two information-
as r=p and r>0 so p>0 so (2/p)>0 , SUFFICIENT

[spoiler]Answer (C)[/spoiler][/quote]

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kul512: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Thu Sep 29, 2011 5:15 amFollowed by: 1 member; Thanked: 7 times; GMAT Score:740

by kul512 » Sat Mar 03, 2012 9:35 pm

myfish what are the OA for these questions? Please post them.

Sometimes there is very fine line between right and wrong: perspective.

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krusta80: Master | Next Rank: 500 Posts; Posts: 143; Joined: Mon Mar 14, 2011 3:13 amFollowed by: 5 members; Thanked: 34 times

by krusta80 » Sat Mar 03, 2012 10:38 pm

Question 1

myfish wrote:I have the right answers from GMATPrep but no idea how they got there, even after 10 min Please take a look at the screenshots and take a crack at it. Thanks a lot!!!!!

Given: 5x + 3y <= 10
4x + 4y <= 10?

Part (1)

x < 1

Let's plug in 1 for x in both formulas to see what happens:

Given: y <= 5/3
y <= 3/2 ?

Clearly this is TRUE.

Now, plugging in 0 for x in both forumulas to see what happens:
y <= 10/3
y <= 10/4?

Not always TRUE... INSUFFICIENT!

Part (2)

11x <= 10
x <= 10/11

Plugging the min value for x into the given equation...

x = 0 -> y <= 10/3

4x + 4y <= 10?
y <= (5-2x)/2

Plugging in 0 for x again...

y <= 5/2 ? NOT NECESSARILY NOT SUFFICIENT

Combining both doesn't help the case when x = 0, which still allows for either case.

Answer is E.

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krusta80: Master | Next Rank: 500 Posts; Posts: 143; Joined: Mon Mar 14, 2011 3:13 amFollowed by: 5 members; Thanked: 34 times

by krusta80 » Sat Mar 03, 2012 10:45 pm

Question 2

2x - 3y < x^2

Part (1)
2x - 3y = -2

Since x^2 can never be less than 0, we know that the main inequality will always hold true.

SUFFICIENT

Part (2)
x > 2 and y > 0

The y > 0 tells us that we will always be subtracting from the 2x, which will be greater than 4. On the right-hand-side, we have x^2, which will also always be greater than 4. Since even at x = 2 the RHS is greater than the left, we know that it ALWAYS will be greater for all values of x > 2, because the RHS will always rise more quickly.

SUFFICIENT

D

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krusta80: Master | Next Rank: 500 Posts; Posts: 143; Joined: Mon Mar 14, 2011 3:13 amFollowed by: 5 members; Thanked: 34 times

by krusta80 » Sat Mar 03, 2012 10:52 pm

Question 3

1/p > r/(r^2+2) ?

Part (1)
p = r

If p and r are positive, then we can see than 1/p will always be greater, because r/r^2 equals 1/p but the extra +2 in the denominator will always make the RHS a touch smaller!

If p and r are negative, however, the fractions become negative, which means that the inverse is true: the RHS will be slightly larger.

INSUFFICIENT

Part (2)
r > 0

This is not enough, because it tells us nothing of p.

Combined, parts (1) and (2) are SUFFICIENT, since now we know that r and p are positive, which always makes the main inequality true.

C

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ranjeet75: Master | Next Rank: 500 Posts; Posts: 242; Joined: Fri Aug 01, 2008 8:49 am; Location: Delhi; Thanked: 6 times

by ranjeet75 » Sun Mar 04, 2012 7:38 am

[quote="krusta80"][size=18]Question 2[/size]

2x - 3y < x^2

Part (1)
2x - 3y = -2

Since x^2 can never be less than 0, we know that the main inequality will always hold true.

SUFFICIENT

Part (2)
x > 2 and y > 0

The y > 0 tells us that we will always be subtracting from the 2x, which will be greater than 4. On the right-hand-side, we have x^2, which will also always be greater than 4. Since even at x = 2 the RHS is greater than the left, we know that it ALWAYS will be greater for all values of x > 2, because the RHS will always rise more quickly.

SUFFICIENT

D[/quote]

Please clarify Part 2 as I could not understand

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krusta80: Master | Next Rank: 500 Posts; Posts: 143; Joined: Mon Mar 14, 2011 3:13 amFollowed by: 5 members; Thanked: 34 times

by krusta80 » Sun Mar 04, 2012 8:55 am

ranjeet75 wrote:
krusta80 wrote:Question 2

2x - 3y < x^2

Part (1)
2x - 3y = -2

Since x^2 can never be less than 0, we know that the main inequality will always hold true.

SUFFICIENT

Part (2)
x > 2 and y > 0

The y > 0 tells us that we will always be subtracting from the 2x, which will be greater than 4. On the right-hand-side, we have x^2, which will also always be greater than 4. Since even at x = 2 the RHS is greater than the left, we know that it ALWAYS will be greater for all values of x > 2, because the RHS will always rise more quickly.

SUFFICIENT

D
Please clarify Part 2 as I could not understand

Sure...so, we are are posed with the question: Is 2x - 3y < x^2?

In part 2, we are told that x > 2 and y > 0.

Let's pretend for the moment that y = 0, which we know is less than the smallest value we can make y in this part but it helps to make my point.

If y = 0, then the question simplifies to:
2x < x^2?

Without getting into heavy algebra, it is pretty simple to see that 2x will always be less than x^2 if x is greater than 2, since 2x = x^2 at x = 2 and since x^2 grows much more quickly than 2x (imagine it drawn out on a graph...or see below).

So if y = 0, then 3y = 0, which means that we take away NOTHING from the 2x on the left hand side of the original equation and we STILL can say that the equation holds true for x > 2. But what happens when y > 0? Well, then 3y > 0, and we end up lessening the value of the left hand side. So clearly, this will NOT change the outcome of the question. It is still always true when y > 0 and x > 2.

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myfish: Senior | Next Rank: 100 Posts; Posts: 67; Joined: Mon May 03, 2010 12:03 pm; Thanked: 3 times

by myfish » Mon Mar 05, 2012 7:09 am

WOW! With real MATLAB graphic! Thanks so much!!!!

[quote="krusta80"][quote="ranjeet75"][quote="krusta80"][size=18]Question 2[/size]

2x - 3y < x^2

Part (1)
2x - 3y = -2

Since x^2 can never be less than 0, we know that the main inequality will always hold true.

SUFFICIENT

Part (2)
x > 2 and y > 0

The y > 0 tells us that we will always be subtracting from the 2x, which will be greater than 4. On the right-hand-side, we have x^2, which will also always be greater than 4. Since even at x = 2 the RHS is greater than the left, we know that it ALWAYS will be greater for all values of x > 2, because the RHS will always rise more quickly.

SUFFICIENT

D[/quote]

Please clarify Part 2 as I could not understand[/quote]

Sure...so, we are are posed with the question: Is 2x - 3y < x^2?

In part 2, we are told that x > 2 and y > 0.

Let's pretend for the moment that y = 0, which we know is less than the smallest value we can make y in this part but it helps to make my point.

If y = 0, then the question simplifies to:
2x < x^2?

Without getting into heavy algebra, it is pretty simple to see that 2x will always be less than x^2 if x is greater than 2, since 2x = x^2 at x = 2 and since x^2 grows much more quickly than 2x (imagine it drawn out on a graph...or see below).

[url=https://postimage.org/image/4g2b9e1vx/][img]https://s14.postimage.org/4g2b9e1vx/graph.jpg[/img][/url]

So if y = 0, then 3y = 0, which means that we take away NOTHING from the 2x on the left hand side of the original equation and we STILL can say that the equation holds true for x > 2. But what happens when y > 0? Well, then 3y > 0, and we end up lessening the value of the left hand side. So clearly, this will NOT change the outcome of the question. It is still always true when y > 0 and x > 2.[/quote]

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krusta80: Master | Next Rank: 500 Posts; Posts: 143; Joined: Mon Mar 14, 2011 3:13 amFollowed by: 5 members; Thanked: 34 times

by krusta80 » Mon Mar 05, 2012 7:16 am

myfish wrote:WOW! With real MATLAB graphic! Thanks so much!!!!

krusta80 wrote:
ranjeet75 wrote:
krusta80 wrote:Question 2

2x - 3y < x^2

Part (1)
2x - 3y = -2

Since x^2 can never be less than 0, we know that the main inequality will always hold true.

SUFFICIENT

Part (2)
x > 2 and y > 0

The y > 0 tells us that we will always be subtracting from the 2x, which will be greater than 4. On the right-hand-side, we have x^2, which will also always be greater than 4. Since even at x = 2 the RHS is greater than the left, we know that it ALWAYS will be greater for all values of x > 2, because the RHS will always rise more quickly.

SUFFICIENT

D
Please clarify Part 2 as I could not understand
Sure...so, we are are posed with the question: Is 2x - 3y < x^2?

In part 2, we are told that x > 2 and y > 0.

Let's pretend for the moment that y = 0, which we know is less than the smallest value we can make y in this part but it helps to make my point.

If y = 0, then the question simplifies to:
2x < x^2?

Without getting into heavy algebra, it is pretty simple to see that 2x will always be less than x^2 if x is greater than 2, since 2x = x^2 at x = 2 and since x^2 grows much more quickly than 2x (imagine it drawn out on a graph...or see below).

So if y = 0, then 3y = 0, which means that we take away NOTHING from the 2x on the left hand side of the original equation and we STILL can say that the equation holds true for x > 2. But what happens when y > 0? Well, then 3y > 0, and we end up lessening the value of the left hand side. So clearly, this will NOT change the outcome of the question. It is still always true when y > 0 and x > 2.

Not a problem (Excel actually)...the whole point was just to help drive home the point of different rates of growth, etc. After enough exposure, things start to become second nature.

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