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Re: Does xy = 1? (1) x > y (2) x = 1/y [#permalink]
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Bunuel wrote:
Does xy = 1?

(1) x > y

(2) x = 1/y


Target question: Does xy = 1?

Statement 1: x > y
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several values of x and y that satisfy statement 1. Here are two:
Case a: x = 2 and y = 0.5. In this case, xy = (2)(0.5) = 1. So, the answer to the target question is YES, xy equals 1
Case b: x = 2 and y = 1. In this case, xy = (2)(1) = 2. So, the answer to the target question is NO, xy does NOT equal 1
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Aside: For more on this idea of testing values when a statement doesn't feel sufficient, read my article: https://www.gmatprepnow.com/articles/dat ... lug-values

Statement 2: x = 1/y
Multiply both sides of the equation by y to get: xy = 1
So, the answer to the target question is YES, xy equals 1
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

ASIDE: Some students may be concerned that I just multiplied both sides of the equation by a variable (y), which opens up the potential of inadvertently multiplying by zero.
However, the equation x = 1/y implies that why cannot equal zero, because 1/0 is undefined (i.e., it cannot be evaluated to be a real number).
So, if 1/0 it's not a real number, then we cannot say that x = 1/0, since that would mean that X is also not a real number


Answer: B

Cheers,
Brent
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Does xy = 1? (1) x > y (2) x = 1/y [#permalink]
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Bunuel wrote:
Does xy = 1?

(1) x > y

(2) x = 1/y



Asked: Does xy = 1?

(1) x > y
There are many possibilities e.g.
1. x= 2 & y=1; xy=2
2. x=2 & y=1/2; xy=1
NOT SUFFICIENT

(2) x = 1/y
xy=1
SUFFICIENT

IMO B

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Does xy = 1? (1) x > y (2) x = 1/y [#permalink]
1
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Expert Reply
Bunuel wrote:
Does xy = 1?

(1) x > y

(2) x = 1/y



Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.
The question \(xy = 1\) is equivalent to \(x = \frac{1}{y}\).

Then, condition 2) is sufficient obviously.

Condition 1)

If \(x = 2, y = \frac{1}{2}\), then we have \(xy = 1\) and the answer is 'yes'.
If \(x = 4, y = \frac{1}{2}\), then we have \(xy = 2\) and the answer is 'no'.

Since condition 1) does not yield a unique solution, it is not sufficient.

Therefore, B is the answer.
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Re: Does xy = 1? (1) x > y (2) x = 1/y [#permalink]
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Re: Does xy = 1? (1) x > y (2) x = 1/y [#permalink]
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