What is the value of y-x^2-x?

Question

(1) y = -3x
(2) y = -4(x + 1)

mansoorfarooqui · Accepted Answer

We need to find whether info is sufficient to find value of y-x2-x.

We cant simplify the question stem further, so lets start with st1:
(1) y = -3x --> two vars 1 eq--> cant solve
(2) y = -4(x + 1) --> two vars 1 eq--> cant solve

1+2 together is solvable.
y=-3x and also y=-4x-4

so -3x=-4x-4
x=-4, y=12
we dont actually need to solve but for illustration i did. Knowing that we can solve them to get unique values of x and y is enough.

So 1 and 2 taken together are sufficient

Best Regards,
Mansoor

PS: Please Consider Kudos if you find it helpful

aeglorre · Answer

Can someone please give a more in depth explanation to this. I would also appreciate if the question was  using markup  because the expression in the question above is rather ambiguous.

Bunuel · Answer

What is the value of  y-x^2-x ?

(1) y = -3x --> the question becomes: what is the value of  -3x-x^2-x . We need the value of x to answer this question. Not sufficient.

(2) y = -4(x + 1) --> the question becomes: what is the value of  -4(x + 1)-x^2-x . We need the value of x to answer this question. Not sufficient.

(1)+(2) We have two distinct linear equations:  y = -3x  and  y = -4(x + 1) , thus we can solve for both variables and get the value of  y-x^2-x . Sufficient.

Answer: C.

Hope it's clear.

aeglorre · Answer

Awesome, thanks a lot Bunuel. You are, as they say, "The Man".

Posted from my mobile device

Raihanuddin · Answer

The explanation about linear equation is not clear

Bunuel · Answer

If you have n distinct linear equations and n variables then you can solve for them. "Distinct linear equations" means that no equation can be derived with the help of others or by arithmetic operation (by multiplication, addition, ...).

We have two distinct linear equations:  y = -3x  and  y = -4(x + 1) . Solving gives: x=-4 and y=12.

Hope it's clear.

EMPOWERgmatRichC · Answer

Hi All,

This DS question can be solved with a combination of TESTing VALUES and Algebra skills.

We're asked for the value of Y - X^2 - X.

Fact 1: Y = -3X

IF....
X = 0
Y = 0
The answer to the question is 0 - 0^2 - 0 = 0

IF...
X = 1
Y = -3
The answer to the question is -3 - 1^2 - 1 = -5
Fact 1 is INSUFFICIENT

Fact 2: Y = -4(X+1)

IF...
X = 0
Y = -4
The answer to the question is -4 - 0^2 - 0 = -4

IF...
X = 1
Y = -8
The answer to the question is -8 - 1^2 - 1 = -10
Fact 2 is INSUFFICIENT

Combined, we know...
Y = -3X
Y = -4(X+1)

Looking at this, you should recognize a "system" of equations - we can set the equations equal to one another...

-3X = -4X - 4
X = -4

With one fixed value for X, we can solve for Y:

Y = -3(-4) = 12

Thus, X = -4 and Y = 12, so we CAN answer the question.
Combined, SUFFICIENT

Final Answer:  C

GMAT assassins aren't born, they're made,
Rich

SonofAnarchy · Answer

Does this question belongs to the 700 level category? Seems pretty straightforward for a 700 leveler...

Bunuel · Answer

The difficulty level is based on user attempts on this question. Check the stats in the original post.

EMPOWERgmatRichC · Answer

Hi dhruvmadra,

As you study, you're going to find that what you think is "hard" is not necessarily what the Test Makers 'define' as "hard" (and vice-versa). It's not practical to spend time trying to assess the difficulty level of any individual question. Since your goal is almost certainly to increase your scores, you should look at the types of questions that you're getting wrong and WHY you're getting them wrong. If you get a question wrong that you SHOULD have gotten correct (but you made a silly/little mistake), then I don't care what 'level' of question it is - you've got to work to keep those mistakes from happening (since too many little mistakes will kill your scores).

GMAT assassins aren't born, they're made,
Rich

pranskius · Answer

Hi Bunuel, love your answers on each thread! Could you please clarify how to easily spot "distinct" linear equations? Does that mean essentially that you can add them or subtract them (or solve one and substitute in another) and in doing so eliminate one of the two variables? Just trying to figure out how to quickly understand if equations are distinct.

For example:
x + y = 100
x - y = 100

These are distinct, correct? y = 0 and x = 100. However, is there a trick for spotting that, or do you just have to quickly mentally almost "solve" them and see?

x + y = 100
2x + 2y = 200

not distinct because one equation is just a multiple of another. Is the trick to try to spot multiples of equations or are there even more advanced traps used by GMAT?

EMPOWERgmatRichC · Answer

Hi pranskius,

To start, nothing about a GMAT question is ever "random" - every word (and in the case of most Quant questions, every number) is specifically chosen to 'test' you on one or more concepts. When you find yourself thinking... this piece of information is a bit 'weird' relative to the other pieces of information that I was given in the prompt, you should take a moment to think/work through the differences.

For example, if you're first given a straight-forward equation such as X + 2Y = 100.... but then the next equation you're given is 4Y = 200 - 2X... you should take a moment to think about WHY the author wrote the second equation in a different way... Maybe it's to 'test' your ability to simplify an equation and spot a pattern?

As a general rule, you should be doing all of your work on your notepad (and not doing any steps "in your head", since you're far more likely to make a silly mistake when you don't take notes). When you re-format that second equation, you might also recognize that all of the numbers are EVEN - meaning that you can divide the entire equation by 2... Doing so gives you X + 2Y = 100. Thus, we do NOT have 2 unique equations here - we have the SAME equation twice (and this is NOT a 'System' of equations). 
 
Most Quant questions require that you work through 3-5 'steps' to get the correct answer. Most prompts are fairly "thin" when it comes to information though, so you should get in the habit of looking for simple steps that you can do to organize and simplify information that you've been given. Going through those steps can actually help save you time and make it easier to spot the built-in pattern(s) involved.

GMAT assassins aren't born, they're made,
Rich

bumpbot · Answer

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What is the value of y-x^2-x?

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