Re: Data Sufficiency Questions
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08 Nov 2011, 07:57
Data sufficiency is one of those things that most newbies love to hate. It's GMAT specific and it gets you because you are looking for what's SUFFICIENT rather than a correct answer
What determines sufficiency?
Before we answer that, we'll have to determine what type of DS question it is. Fortunately, there are only two types
(1) Yes/No
(2) Value
Yes/No questions will look for an answer that is a DEFINITE yes or DEFINITE NO -> this determines SUFFICIENCY. If one of your statements produces a COULD BE OR sometimes yes/sometimes no, then it is INSUFFICIENT.
Value questions ask for ONE VALUE, if your answer produces ONE VALUE then it is sufficient. If it produces more than one value, then you can bet yourself it is INSUFFICIENT (this will become more complicated with boundaries)
It's important to remember your answer choices as well
A - ONLY statement 1 is sufficient
B - ONLY statement 2 is sufficient
C - statement 1 and 2 TOGETHER are sufficient, but not by themselves
D - statement 1 and 2 are BOTH sufficient INDEPENDENT of each other
E - NEITHER STATEMENT is sufficient, together or independent of each other
Let's do an example:
If x is positive, is x a prime number?
(1) x is odd.
(2) x < 8
your first step is to find out what kind of question this is.
The question is "is x a prime number?" what would your response be? "hmm I would either say Yes or no this question"
Your next step is translate the english into math
So, X is positive. the question remains...is X a prime number, (in other words) is X=2,3,5,7,9 .... etc
Well we can solve the problem by looking at the statements.
Let's look at (1) first
It tells us X is odd. Well that means X can be 3,5,7,9,11,13,15...etc.
OK so 3 is prime, 5 is prime...that means yes right?
NO, check numbers that you think MIGHT give us a no. let's choose 9, 9 is NOT prime AND it is odd so that answer is NO
So what does this mean? 3 gives us a YES answer and 9 gives us a NO
that means statement 1 is INSUFFICIENT. We need a definite yes or definite no.
Let's hop down to statement 2
It tells us x < 8.
From the Question STEM X is positive, so 0<x<8 applies meaning x is between 0 and 8.
What really matters is that X = 1,2,3,4,5,6,7
Let's do the same test as statement 1.
Is 1 prime? No
Is 2 prime? Yes
STOP
that gives us a Yes/No so it is INSUFFICIENT
Next we move to choice C and take the statements together.
So X<8 AND x is odd
So that means X has the values of 1,3,5,7.
Is 1 prime? No
Is 3 prime? Yes
STOP
this is INSUFFICIENT.
So now you are left with choice E (because since both statement 1 and 2 are not sufficient independently we can't choose D)
IN addition, look up videos on youtube and punch in Data sufficiency, the thought process is hard to comprehend because you really aren't used to it. Best of luck!