XY

[rkhicha]

Is y - x positive?
(1) y > 0
(2) x = 1 - y

Have doubts on the answer provided by the book.[/spoiler]

[gmatmachoman]

Is y - x positive?
(1) y > 0
(2) x = 1 - y


after rephrasing : is K= y-x>0

st 1 : y>0

Gives no idea abt X. So insufficient

st 2 :

x= 1-y
applying this in the stem; y -(1-y)

------ : y-1+y

= 2y -1

Depending up on the value of y, the output will change. Fro example say Y = 1/2, then K =0

or itcan be like , Y = 2 , then K = 3 which is greater than 0.
Inconsistent . So Insufficient

After combining, y>0 and X= 1-y we will NOT arrive to solid conclusion

when y = 1/4 ( remember Y is >0)
K = 2y-1
= 2 * 1/4 -1
-1/2 which is less than 0
when Y = 4

K = 2*4 -1
=7 >0
Inconsistent as we see K is less than/equalto/greater than zero even after using 2 sts..

So pick E

[Stuart@KaplanGMAT]

Is y - x positive?
(1) y > 0
(2) x = 1 - y

Have doubts on the answer provided by the book.[/spoiler]

Let's rephrase the question a bit differently:

Is y - x > 0?

Is y > x?

(1) as noted, no info about y... insufficient, eliminate A and D.

(2) x = 1 - y

plugging into the question:

Is y > 1 - y?

Is 2y > 1?

Is y > 1/2?

No clue! Insufficient, eliminate B.

Together:

(1) y > 0.

From 2, our question is now: Is y > 1/2?

Maybe yes, maybe no. We could pick y = 100 to get a "yes" answer or y = 1/4 to get a "no" answer.

Not enough information: choose (E).