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vscid: Master | Next Rank: 500 Posts; Posts: 212; Joined: Sat Dec 01, 2007 4:19 pm; Thanked: 5 times

by vscid » Sat Feb 20, 2010 3:47 pm

If x2/9 - 4/y2 = 12, what is the value of x?

(1) x/3 + 2/y = 6

(2) x/3 - 2/y = 2

The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.

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Source: Beat The GMAT — Data Sufficiency | Sat Feb 20, 2010 3:47 pm

onedayi'll: Senior | Next Rank: 100 Posts; Posts: 87; Joined: Wed Jan 06, 2010 8:16 pm; Location: Chandigarh, INDIA

by onedayi'll » Sat Feb 20, 2010 3:58 pm

IMO C

Gym/GMAT/Girl -- Be Serious

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thephoenix: Legendary Member; Posts: 1560; Joined: Tue Nov 17, 2009 2:38 amFollowed by: 5 members; Thanked: 137 times

by thephoenix » Sat Feb 20, 2010 6:14 pm

vscid wrote:If x2/9 - 4/y2 = 12, what is the value of x?

(1) x/3 + 2/y = 6

(2) x/3 - 2/y = 2

s1)squaring bth side=x^2/9+4/y^2+x^2/18y=36
but we dnt know x andy
insuff
s2) same as s1)
insuff

combining
multiplying bth statements
x^2/9-4/y^2=12 same as the eqn in q
hence two variables and one eqn----> no solution
hence insuff

IMO E

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Sat Feb 20, 2010 8:38 pm

vscid wrote:If x2/9 - 4/y2 = 12, what is the value of x?

(1) x/3 + 2/y = 6

(2) x/3 - 2/y = 2

x^2/9 - 4/y^ 2 = 12 --> (x/3 )^2- (2/y)^2 = 12 -->(x/3 + 2/y)(x/3 - 2/y) = 12

s.(1) x/3 + 2/y = 6 ---> (x/3 - 2/y) = 12/6 =2 two eq we can solve it ===== sufficient.
s.(2) x/3 - 2/y = 2 ---> (x/3 + 2/y) = 12/2 =6 two eq we can solve it ===== sufficient.

Ans D

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Sat Feb 20, 2010 8:40 pm

thephoenix wrote:
vscid wrote:If x2/9 - 4/y2 = 12, what is the value of x?

(1) x/3 + 2/y = 6

(2) x/3 - 2/y = 2

s1)squaring bth side=x^2/9+4/y^2+x^2/18y=36
but we dnt know x andy
insuff
s2) same as s1)
insuff

combining
multiplying bth statements
x^2/9-4/y^2=12 same as the eqn in q
hence two variables and one eqn----> no solution
hence insuff

IMO E

Phoenix I think u did a mistake..In this type of problem don't go on with squaring like
x/3 + 2/y = 6
x^2/9+4/y^2+x^2/18y=36

it will lead you in confusion and u will not be able to solve the problem..

Rather try to divide the given expression so thjat u can get simpler terms
x^2/9 - 4/y^ 2 = 12 --> (x/3 )^2- (2/y)^2 = 12 -->(x/3 + 2/y)(x/3 - 2/y) = 12

In this way u can easily reach to the Answer ...

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thephoenix: Legendary Member; Posts: 1560; Joined: Tue Nov 17, 2009 2:38 amFollowed by: 5 members; Thanked: 137 times

by thephoenix » Sat Feb 20, 2010 8:44 pm

shashank.ism wrote:
thephoenix wrote:
vscid wrote:If x2/9 - 4/y2 = 12, what is the value of x?

(1) x/3 + 2/y = 6

(2) x/3 - 2/y = 2

s1)squaring bth side=x^2/9+4/y^2+x^2/18y=36
but we dnt know x andy
insuff
s2) same as s1)
insuff

combining
multiplying bth statements
x^2/9-4/y^2=12 same as the eqn in q
hence two variables and one eqn----> no solution
hence insuff

IMO E
Phoenix I think u did a mistake..In this type of problem don't go on with squaring like
x/3 + 2/y = 6
x^2/9+4/y^2+x^2/18y=36

it will lead you in confusion and u will not be able to solve the problem..

Rather try to divide the given expression so thjat u can get simpler terms
x^2/9 - 4/y^ 2 = 12 --> (x/3 )^2- (2/y)^2 = 12 -->(x/3 + 2/y)(x/3 - 2/y) = 12

In this way u can easily reach to the Answer ...

thanks dost

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Sat Feb 20, 2010 8:45 pm

thephoenix wrote:
thanks dost

anytime phoenix....u could have thanked me through thanks button...anyway its fine..

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vscid: Master | Next Rank: 500 Posts; Posts: 212; Joined: Sat Dec 01, 2007 4:19 pm; Thanked: 5 times

by vscid » Mon Feb 22, 2010 2:09 pm

OA is D.

The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.

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lunarpower: GMAT Instructor; Posts: 3380; Joined: Mon Mar 03, 2008 1:20 amFollowed by: 1535 members; Thanked: 2256 times; GMAT Score:800

by lunarpower » Tue Feb 23, 2010 6:21 am

there are some wrong answers in this thread.

you should be able to recognize the difference of squares pattern INSTANTLY. if you don't recognize this pattern in a few seconds, you should put it on a flash card, and drill it like crazy.

the expression x^2/9 - 4/y^2 is a difference-of-squares. you should factor it before you even turn on your brain!
so:
(x/3 - 2/y)(x/3 + 2/y) = 12.

if you have statement (1), you can DIVIDE this equation by statement (1), giving ... statement (2)!
if you have statement (2), you can DIVIDE this equation by statement (2), giving ... statement (1)!
so, basically, if you have either of the statements, you have them both.

if you have both statements, you can just line them up and add them, giving 2x/3 = 8. (the y's cancel.)
that's good enough, so the answer is (d).

Ron has been teaching various standardized tests for 20 years.

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sairamGmat: Senior | Next Rank: 100 Posts; Posts: 39; Joined: Thu Feb 25, 2010 7:04 am; Thanked: 2 times

by sairamGmat » Thu Feb 25, 2010 8:19 am

D is the answer. The question is in the form of a^2-b^2 = <some constant> . so, it can be rewritten as (a+b) (a-b)

In both options, one of these , viz (a+b) and (a-b) are given. So, we can calculate the other word and hence a or b by using either of the options..

so, Answer is D

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