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If y=3x, what is the value of x+y?

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Intern
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If y=3x, what is the value of x+y?  [#permalink]

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New post 16 Sep 2018, 08:05
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Question Stats:

92% (00:49) correct 8% (01:08) wrong based on 38 sessions

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If y=3x, what is the value of x+y?

1) 3y=4x+21

2) x<y

Source: GMAT Prep Course, Jeff Kolby
Intern
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Re: If y=3x, what is the value of x+y?  [#permalink]

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New post 16 Sep 2018, 09:25
x+y can also be written as x+3x = 4x

1) 3y=4x+21
Substituting y with 3x in the equation

9x = 4x+21, therefore we can find the value of x and thus 4x or x+y - Sufficient

2) x<y
This does not give us unique values for x and y

if x = 1, then y = 3, and x+y=4

if x = 2, then y = 6, and x+y=8

x+y has no unique value - Insufficient

Answer: A
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Re: If y=3x, what is the value of x+y?  [#permalink]

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New post 17 Sep 2018, 10:18
Top Contributor
bettatantalo wrote:
If y = 3x, what is the value of x+y?

1) 3y = 4x + 21

2) x < y


Target question: What is the value of x+y?

Given: y = 3x

Statement 1: 3y = 4x + 21
We also know that y = 3x

Since we have two DIFFERENT linear equations with 2 variables, we COULD solve the system of equations for x and y, which means we COULD determine the value of x+y (Of course, we'd never waste valuable time actually solving the system)
Since we COULD answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: x < y
There are several values of x and y that satisfy statement 2 (And satisfy the given information). Here are two possible cases:
Case a: x = 1 and y = 3. In this case, the answer to the target question is x + y = 4
Case b: x = 2 and y = 6. In this case, the answer to the target question is x + y = 8
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: B

Cheers,
Brent
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Re: If y=3x, what is the value of x+y? &nbs [#permalink] 17 Sep 2018, 10:18
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