help!!!

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jayanti: Senior | Next Rank: 100 Posts; Posts: 72; Joined: Tue Apr 19, 2011 6:58 amFollowed by: 6 members

by jayanti » Fri Aug 12, 2011 7:52 am

The operation * is defined by the equation x*y = (x-y)/(x+y), where y â‰ -x. If 3*y = 5*4,then y =
A. 1/9
B. 4/15
C. 5/12
D. 12/5
E. 15/4

I cant figure this one out.......

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Source: Beat The GMAT — Problem Solving | Fri Aug 12, 2011 7:52 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Fri Aug 12, 2011 9:06 am

jayanti wrote:The operation * is defined by the equation x*y = (x-y)/(x+y), where y â‰ -x. If 3*y = 5*4,then y =

Given that, x*y = (x-y)/(x+y)

Thus, 3*y = (3 - y)/(3 + y)
And, 5*4 = (5 - 4)/(5 + 4) = 1/9

So, (3 - y)/(3 + y) = 1/9
--> 9(3 - y) = (3 + y)
--> (27 - 9y) = (3 + y)
--> 10y = 24
--> y = 24/10 = 12/5

The correct answer is D.

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jayanti: Senior | Next Rank: 100 Posts; Posts: 72; Joined: Tue Apr 19, 2011 6:58 amFollowed by: 6 members

by jayanti » Fri Aug 12, 2011 10:03 am

Hi Sir, Could you pls explain the

3*y = (3 - y)/(3 + y)
And, 5*4 = (5 - 4)/(5 + 4) = 1/9

I dont seem to get it:(

Thanks a lot...

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Fri Aug 12, 2011 10:15 am

jayanti wrote:Hi Sir, Could you pls explain the

3*y = (3 - y)/(3 + y)
And, 5*4 = (5 - 4)/(5 + 4) = 1/9

I don't seem to get it:(

Thanks a lot...

The question defines the operation x*y as (x - y)/(x + y).
Hence, a*b = (a - b)/(a + b)
Similarly, 5*4 = (5 - 4)/(5 + 4)
And, 3*y = (3 - y)/(3 + y)

To put it simply replace x with left operand and replace y with right operand in the expression.

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