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When you plug 3 back into 3y + y = 18, why does it not equal 18? I got the question right on my mock, I just want to keep from getting confused. Is it just the fact that we COULD solve for y?
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tripledown
When you plug 3 back into 3y + y = 18, why does it not equal 18? I got the question right on my mock, I just want to keep from getting confused. Is it just the fact that we COULD solve for y?

Hello

Yes, its the fact that we can solve for y and get a unique value of y - that combined the two statements are sufficient to answer the question. Thats why the answer is C.
Value of y is immaterial but still: 3y+y = 18 means y = 18/4.
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Bunuel
If x is positive, what is the value of y ?

(1) 5x = 15 --> x = 3. We know nothing about y. Not sufficient.#

(2) xy + y = 18. Infinitely many pairs of (x, y) satisfy this. For example, x = 1 and y = 9; x = 2 and y = 6; ... Not sufficient.

(1)+(2) Since x = 3 from (1), then y = 3 from (2). Sufficient.

Answer: C.

P.S. Is this a new question from GMAT Prep?

Hi Bunuel,
I am getting a different answer for y. Can you help me understand why
x= 3
the y(x+1)=18
then
y(4)=18
then y= \(\frac{18}{4}\)
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Bunuel
If x is positive, what is the value of y ?

(1) 5x = 15 --> x = 3. We know nothing about y. Not sufficient.#

(2) xy + y = 18. Infinitely many pairs of (x, y) satisfy this. For example, x = 1 and y = 9; x = 2 and y = 6; ... Not sufficient.

(1)+(2) Since x = 3 from (1), then y = 3 from (2). Sufficient.

Answer: C.

P.S. Is this a new question from GMAT Prep?

Hi Bunuel,
I am getting a different answer for y. Can you help me understand why
x= 3
the y(x+1)=18
then
y(4)=18
then y= \(\frac{18}{4}\)

Yes, you are right. Edited the typo. Thank you.
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