Brent@GMATPrepNow wrote:
Is AB || CD?

(1) AC = BD
(2) AB = CD

Answer: E
Difficulty level: 600 - 650
Source: www.gmatprepnow.com

*I'll post a full solution in 2 days

Target question: Is AB || CD?
I'll use the two shapes below (rectangle and isosceles trapezoid) show that the correct answer is E


Statement 1: AC = BD
Notice that rectangle and isosceles trapezoid both satisfy the condition that AC = BD

In the case of the rectangle, the answer to the target question is YES, AB IS parallel to CD
In the case of the isosceles trapezoid, the answer to the target question is NO, AB is NOT parallel to CD
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: AB = CD
Notice that rectangle and isosceles trapezoid both satisfy the condition that AB = CD

In the case of the rectangle, the answer to the target question is YES, AB IS parallel to CD
In the case of the isosceles trapezoid, the answer to the target question is NO, AB is NOT parallel to CD
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
IMPORTANT: Notice that I was able to use the same counter-examples to show that each statement ALONE is not sufficient.
So, the same counter-examples must also satisfy the two statements COMBINED.
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer: E

Cheers,
Brent