The way the question is given doesn't make sense, because if 1/x=1/y, then x=y, then (1) becomes 1/2 = -1 and GMAT will never give you a wrong statement in DS.

tutorphd wrote:The way the question is given doesn't make sense, because if 1/x=1/y, then x=y, then (1) becomes 1/2 = -1 and GMAT will never give you a wrong statement in DS.

I was very confused with this question. Thank you for pointing this out. This is from an online source called GWD. My Chinese friend helped my find it. They are supposed to be past GMAT questions. Most questions are pretty good, but sometimes there are problems. My apologies

alex.gellatly wrote:what is the value of (1/x) - (1/y)?
1. x/(x+y)=-1
2. xy=6(x+y)

Thanks

Question seems fine. I think the wordings are:

What is the value of (1/x) - (1/y)?
(1) x/ (x + y) = -1.
(2) x y = 6 (x + y).

Statement 1 results in 2x = -y or y = -2x, which makes (1/x) - (1/y) = (1/x) + {1/(2x)} = 3/(2x). Insufficient

Statement 2 gives the value of (1/x) + (1/y) equal to 1/6, and with that we cannot answer (1/x) - (1/y) uniquely. Insufficient

Taking the two statements together, we can have

x/ (x + y) = 6/y = -1, that gives y = -6, and hence x = 3; and finally (1/x) - (1/y) [spoiler]can be answered uniquely as equal to 1/3 + 1/6 = ½.

Win C[/spoiler]