Data Suff - "Is y=3?" / "If p is a prime # gr

[krithika1993]

Hi there,

I was trying some practice questions online when I came across 2 tough ones.

QUESTION 1

1. Is y = 3?
(1) (y - 3)(x - 4) = 0
(2) (x - 4) = 0
Correct Answer: (E).

I'm not sure how they determined the answer to be E. I think it should be A because in statement 1, if each factor is set equal to 0, solving for y in the first factor gives you 3. Solving for x gives you 4 (but this is irrelevant to the question being asked).

QUESTION 2

If p is a prime number greater than 2, what is the value of p?
(1) There are a total of 100 prime numbers between 1 and p+1
(2) There are a total of p prime numbers between 1 and 3912
Correct Answer: (D).

I didn't really know where to begin with this question. I tend to get overwhelmed/intimidated by questions that provide very large numbers of values between 1 value and the other (i.e. 100 prime numbers between 1 and p+1). Would there be an efficient approach that I can use for all such questions (whether or not I am dealing with prime/non prime #s)?

Please let me know where I'm misinterpreting the information/how I could improve on such "solve for this variable" data sufficiency problems.

Thank you very much!
Krithika

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Hi krithika1993,

For future reference, you should post just one question per thread. In that way, you can keep each conversation focused on just one question and avoid the confusion of multiple conversations about different questions occurring in the same place. In addition, you should use the spoiler 'button' to hide the correct answer so that other users can attempt the question on their own before revealing what the correct answer actually is).

For the first question, we're asked if Y = 3. This is a YES/NO question.

1) (Y-3)(X-4) = 0

With this Fact, one (or both) of the following must be true: Y = 3 and/or X = 4. If X=4, then it does not matter what Y is (it COULD be 3, but it could also be any other number), so the answer to the question could be YES or NO.
Fact 1 is INSUFFICIENT

2) (X-4) = 0

This proves that X = 4, but it tells us nothing about the value of Y.
Fact 2 is INSUFFICIENT

Combined, we know that X = 4, but we still don't know anything about the value of Y.
Combined, INSUFFICIENT

Final Answer: E

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

[krithika1993]

Oops! Will keep what you said in mind for future posts - sorry about that; I'm a new member to the site.

Thank you very much for the help by the way. Your explanation makes complete sense. Appreciate it!