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Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0
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Updated on: 24 Jun 2018, 10:59

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Bunuel wrote:

Is \(\frac{x + 1}{y + 1} > \frac{x}{y}\)?

(1) \(0 < x < y\)

(2) \(xy > 0\)

NEW question from GMAT® Official Guide 2019

(DS09315)

We'll simplify the inequalities so we understand what we need to do. This is a Precise approach.

(1) Multiplying both sides by y(y+1) (note that both are positive), gives yx+y > yx+x and cancelling out yx gives y > x. This is exactly what we're told in our statement! Sufficient.

(2) If y(y+1) is positive we can repeat the same process as above to get y > x, but since all we know is that both x,y are positive or both are negative but do not know which is larger we cannot answer. If y(y+1) is negative we'll get the expression y < x, but, for the same reason as above, we still cannot answer. Insufficient

The rephrased question now reads \(\frac{x+y}{y(y+1)} > 0\)

(1) \(0 < x < y\) Since x and y are both positive, the expression will always be positive. (Sufficient)

(2) \(xy > 0\) Here, both x and y can be positive or negative. If x and y are negative, the expression is NOT positive. We don't get a unique answer. (Insufficient)(Option A) _________________

You've got what it takes, but it will take everything you've got

Re: Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0
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24 Jun 2018, 14:33

Top Contributor

Bunuel wrote:

Is \(\frac{x + 1}{y + 1} > \frac{x}{y}\)?

(1) \(0 < x < y\)

(2) \(xy > 0\)

Target question:Is (x+1)/(y+1) > x/y ?

Statement 1: 0 < x < y This tells us that y is POSITIVE, which means y+1 is also POSITIVE. This means we can safely take the inequality (x+1)/(y+1) > x/y and safely multiply both sides by y When we do so, we get: (y)(x+1)/(y+1) > x We can also multiply both sides by y+1 to get: (y)(x+1) > (x)(y+1) Expand to get: xy + y > xy + x Subtract xy from both sides to get: y > x So, with the help of statement 1, our original target question Is (x+1)/(y+1) > x/y ? becomes Is y > x ? Since statement 1 tells us that y > x, the answer to the target question is a definitive YES Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: xy > 0 Let's TEST some values There are several values of x and y that satisfy statement 2 (xy > 0). Here are two: Case a: x = 1 and y = 1. In this case, (x+1)/(y+1) = (1+1)/(1+1) = 2/2 = 1, and x/y = 1/1 = 1. So, the answer to the target question is NO, (x+1)/(y+1) is NOT greater than x/y ? Case b: x = -3 and y = -2. In this case, (x+1)/(y+1) = (-3 +1)/(-2 +1) = -2/-1 = 2, and x/y = -3/-2 = 3/2. So, the answer to the target question is YES, (x+1)/(y+1) IS greater than x/y ? Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

How can we cross multiply in this question, when we are not aware of the signs of x and y?

Warm Regards, FANJ

Hey FANewJersey, good question! In 1) we do know the sign: we are told that 0 < x < y - this means they are both bigger than 0 - they are positive!
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Re: Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0
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25 Oct 2018, 04:23

Hi DavidTutorexamPAL, So basically you are reading ahead to get an idea and then applying something from the statements to firm up your stem question (is x less than y). Correct? Warmly, FANJ

Re: Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0
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25 Oct 2018, 05:06

FANewJersey wrote:

Hi DavidTutorexamPAL, So basically you are reading ahead to get an idea and then applying something from the statements to firm up your stem question (is x less than y). Correct? Warmly, FANJ

How can we cross multiply in this question, when we are not aware of the signs of x and y?

Warm Regards, FANJ

Hey FANewJersey, good question! In 1) we do know the sign: we are told that 0 < x < y - this means they are both bigger than 0 - they are positive!

You could say that, I suppose... I don't think of it as "reading ahead" since solving with statement (1) is the first thing we are supposed to try and do anyway. The question stem on its own isn't going to give us the answer....
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Re: Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0
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27 Oct 2018, 14:44

Thanks DavidTutorexamPAL, But we will have to nail down a goal, which we drive from the stem question (value vs. Yes/No, what is being asked etc. etc.). But I believe I got the idea here. Thanks

gmatclubot

Re: Is (x + 1)/(y + 1) > x/y? (1) 0 < x < y (2) xy > 0 &nbs
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27 Oct 2018, 14:44