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Y Intercept Of Line

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Source: — Data Sufficiency |

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Re: Y Intercept Of Line

by Brent@GMATPrepNow » Sat Mar 28, 2009 10:32 am
Sumit69 wrote:In the xy plane what is the Y intercept of line L
(i) The slope of line l is 3 times its Y intercept
(ii) The x intercept of line l is -1/3.

Pl help me solve this.
(1) Lots of possible lines
(2) Lots of possible lines

Note: If we draw any (non-vertical) line through the point (-1/3, 0) [aka the x-intercept) the slope of the line will be three times its y-intercept.
Choose any point on the y-axis and determine the slope between this point (-1/3, 0). You'll see that when you divide by the rise (the y-intercept) by the run (1/3), you are essentially muliplying the y-intercept by 3.
I've attached a graphic with 2 possible lines, but there are many (infinitely) more.
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by bec.amit » Sun Mar 29, 2009 3:36 pm
OA C:

what is the answer?
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by iwant700plus » Sun Mar 29, 2009 5:58 pm
y=mx+b

Question: What is b?

1) m=3b - insufficient
2) x intercept =-1/3 - y intercept could be positive or negative depending on the slope of the line. Insufficient.

Together: we have a point x intercept (-1/3,0), plug it into the y=mx+b equation:

0=m(-1/3)+b

Using stmt 1--> 0=3b(-1/3)+b - we can solve for b. Sufficient.

Ans is C.
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by Ramp » Sun Mar 29, 2009 7:26 pm
Statement 1 and 2 both lead to m=3b so can someone explain why the answer is C .I think it should be E.
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by cm47323 » Sun Mar 29, 2009 11:12 pm
iwant700plus wrote: Using stmt 1--> 0=3b(-1/3)+b - we can solve for b. Sufficient.

Ans is C.
This did not solve for b since b just fell out and you're left with 0=0. What exactly is b? While unnecessary to solve for the answer in DS problems, solving for it is the definition of a proof. That can't be done here. The answer is E.
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by iwant700plus » Mon Mar 30, 2009 4:07 am
You are right. Thanks for catching that!
cm47323 wrote:
iwant700plus wrote: Using stmt 1--> 0=3b(-1/3)+b - we can solve for b. Sufficient.

Ans is C.
This did not solve for b since b just fell out and you're left with 0=0. What exactly is b? While unnecessary to solve for the answer in DS problems, solving for it is the definition of a proof. That can't be done here. The answer is E.
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by dwilliams05 » Tue Mar 31, 2009 1:19 pm
iwant700plus was dead on in the equation but he set the equation equal to 0. it should be y = 3b(-1/3) + b (taking S1 and S2 together of course). so y = 0 (which is ends up being the y-intercept). please correct me if i am wrong?
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