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x in terms of y

by sanju09 » Fri Apr 08, 2011 1:37 am
If (x + 3)/(x - 3) = y, what is the value of x in terms of y?
(A) 3 - y
(B) 3/y
(C) (2 + y)/(y - 2)
(D) (-3y -3)/(1 - y)
(E) 3y/2
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by HSPA » Fri Apr 08, 2011 1:50 am
by componendo and dividendo

x/3 = (y+1) / (y-1)
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by Brent@GMATPrepNow » Fri Apr 08, 2011 3:54 am
sanju09 wrote:If (x + 3)/(x - 3) = y, what is the value of x in terms of y?
(A) 3 - y
(B) 3/y
(C) (2 + y)/(y - 2)
(D) (-3y -3)/(1 - y)
(E) 3y/2
(x + 3)/(x - 3) = y
Multiply both sides by (x-3) --> x+3 = xy-3y
Get the x's on one side --> x - xy = -3y - 3
Factor the left-hand-side --> x(1-y) = -3y - 3
Divide both sides by (1-y) --> x = (-3y-3)/(1-y) --> (D)
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by artistocrat » Fri Apr 08, 2011 9:49 am
You can use substitution if you are not comfortable with algebra (But its important to be comfortable with algebra, of course).

I chose x = 4, so (x + 3)/(x - 3) = y = 7

(-3*7-3)/(1-7) = -24/-6 = 4

Bingo! D.
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