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DS Question (For all inquisitive minds)

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by mketron » Sat Aug 17, 2013 5:31 pm
Ok, so I came across this official question and am very surprised by the answer and no explanation is available.
_____________________________________

Is ax = 3 - bx?

(1) x(a+b) = 3
(2) a = b = 1.5 and x = 1.


As far as I can surmise, statement (1) is the same as the question stem, so BCE. (2) seems to be correct because I calculate 1.5 = 1.5 TRUE. I thought (B); however, the answer key states (D) is the correct answer. Could someone please help?

Thank you!
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Source: — Data Sufficiency |
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by Java_85 » Sat Aug 17, 2013 6:24 pm
I don't know why you said BCE?
1) is enough for answering the question "Is ax=3-bx?", and the answer is YES ==> AD
2) by itself is also enough to answer the Question ==> D

Please let me know if you think I'm wrong.
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by Brent@GMATPrepNow » Sun Aug 18, 2013 6:07 am
mketron wrote: Is ax = 3 - bx?
(1) x(a+b) = 3
(2) a = b = 1.5 and x = 1
Target question: Is ax = 3 - bx?
We can take the equation ax = 3 - bx, and rearrange the terms to get: ax + bx = 3.
So, let's rephrase the target question. . .

Rephrased target question: Is ax + bx = 3?

Statement 1: x(a+b) = 3
Expand this to get: ax + bx = 3
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: a = b = 1.5 and x = 1
Since we're given all three variables (a, b and x), we definitely have enough information to determine whether or not ax + bx = 3
So, statement 2 is SUFFICIENT

Answer = D

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by [email protected] » Sun Aug 18, 2013 12:58 pm
Hi mketron,

In DS questions, it's important to separate the QUESTION from the FACTS.

As a simple example...

Question: Is the car that I'm pointing at red?

Notice how that's just a question; we don't know if the car is red or not.

1) The car is red.

This FACT is enough information to answer the question, so Fact 1 is SUFFICIENT

Now, to this DS question....

Question: Is ax = 3 - bx?

We don't know anything about the variables a, b and x.

1) x(a + b) = 3

This FACT gives us an equation to work with. With some algebra, we get...

ax + bx = 3
ax = 3 - bx

We now KNOW that ax = 3 - bx, so this Fact is SUFFICIENT to answer the question.

The GMAT will throw a couple of DS questions at you in which the information needed to answer the question has been "rearranged" using math rules. Be prepared to play around with the facts and prove whether the info is sufficient or insufficient.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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