BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

ABCD

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 2789
Joined: Tue Jul 26, 2011 12:19 am
Location: Chennai, India
Thanked: 206 times
Followed by:43 members
GMAT Score:640

ABCD

by GmatKiss » Fri Oct 28, 2011 1:03 pm
Given that A = 3y + 8x, B = 3y - 8x, C = 4y + 6x, and D = 4y - 6x, what is the value of x*y?

1)AB + CD = -275
2)AD - BC = 420
Top
Source: — Data Sufficiency |
Legendary Member
Posts: 966
Joined: Sat Jan 02, 2010 8:06 am
Thanked: 230 times
Followed by:21 members

by shankar.ashwin » Fri Oct 28, 2011 7:26 pm
BIMO

(1) gives you 2 possibilities.
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 489
Joined: Tue Jul 05, 2011 11:10 am
Thanked: 28 times
Followed by:5 members

by gmatblood » Wed Nov 02, 2011 1:30 pm
shankar.ashwin wrote:BIMO

(1) gives you 2 possibilities.
Can someone elaborate on this!?

Thanks.
Top
User avatar
GMAT Instructor
Posts: 349
Joined: Wed Sep 28, 2011 3:38 pm
Location: Austin, TX
Thanked: 236 times
Followed by:54 members
GMAT Score:770

by GmatMathPro » Wed Nov 02, 2011 1:50 pm
gmatblood wrote:
shankar.ashwin wrote:BIMO

(1) gives you 2 possibilities.
Can someone elaborate on this!?

Thanks.
For statement 1, if you plug in the values of A,B,C, and D, multiply it out and simplify everything, you get 4x^2-y^2=9. Factoring, we get (2x+y)(2x-y)=9. This will be true if 2x-y=9 and 2x+y=1. Solving this system gives us x=5/2 and y=4 and x*y=10. Or it could be that 2x-y=3 and 2x+y=3. Solving this system gives us x=3/2 and y=0 or x*y=0. There are infinitely many other possibilities for x*y, as there is no reason to restrict our attention to integer factors of 9, but this is enough to prove statement 1 insufficient.

For statement 2, if you plug everything in and simplify, a lot of things cancel out and you end up with x*y=15. Hence, regardless of the values of x and y, their product is always 15. Hence, statement 2 is sufficient.

Ans: B
Pete Ackley
GMAT Math Pro
Free Online Tutoring Trial
Top
Post new topic Post Reply
• Page 1 of 1