Hi all,
I was solving question #41 of the OG13 and don't understand the solutions...
The question asks:
Is 4^(x+y)=8^10?
(1) xy=9
(2) y/x=1/4
The answer is C
My solution was:
1. Restate the question into: 2^2(x+y)=2^30 which equals x+y=15
2. Looked at statement 1 which i restated into: x=9+y
3. I substituted step 2 into step 1 and got x=12 and y=3, therefore statement 1 should be sufficient?
4. I checked whether statment 2 is sufficient. Statement 2 was transformed to x=4y, which I substituted into the question and got 4y+y=15 equals y=3 and x=12. Therefore I thought that statement 2 is also sufficient. Thus I went for D.
Why is it not D?
Thank you for your answers!
OG13 Question #41 on page 278
This topic has expert replies

 Newbie  Next Rank: 10 Posts
 Posts: 2
 Joined: 09 Oct 2012
 neelgandham
 Community Manager
 Posts: 1060
 Joined: 13 May 2011
 Location: Utrecht, The Netherlands
 Thanked: 318 times
 Followed by:52 members
Let me rephrase the question.
Is 4^(x+y)=8^10
Is 2^2(x+y)=2^30
Is (x+y) = 15.
If y = 1, then x = 1+9 = 10. x+y = 11.
If y = 3, then x = 3+9 = 12. x+y = 15.
If y = 1, then x = 4. x+y = 5.
If y = 3, then x = 12. x+y = 15.
x = y + 9
So, 3y = 9 and y = 3. x = 12. x+y = 15
Answer C
Do you now know where you went wrong ? If No, please let me know.
Is 4^(x+y)=8^10
Is 2^2(x+y)=2^30
Is (x+y) = 15.
If x = y + 9,then x + y need not necessarily be 15.(1) xy=9
If y = 1, then x = 1+9 = 10. x+y = 11.
If y = 3, then x = 3+9 = 12. x+y = 15.
x = 4y,then x + y need not necessarily be 15.(2) y/x=1/4
If y = 1, then x = 4. x+y = 5.
If y = 3, then x = 12. x+y = 15.
x = 4y andFrom 1 + 2
x = y + 9
So, 3y = 9 and y = 3. x = 12. x+y = 15
Answer C
Do you now know where you went wrong ? If No, please let me know.
Anil Gandham
Welcome to BEATtheGMAT  Photography  Getting Started  BTG Community rules  MBA Watch
Check out GMAT Prep Now's online course at https://www.gmatprepnow.com/
Welcome to BEATtheGMAT  Photography  Getting Started  BTG Community rules  MBA Watch
Check out GMAT Prep Now's online course at https://www.gmatprepnow.com/

 Newbie  Next Rank: 10 Posts
 Posts: 2
 Joined: 09 Oct 2012
Thank you for your answer, Neel.
I understand that if I'm looking at each statement, without considering x+y=15 from the question stem, then there are more than one possible solution. However, I included x+y=15 from the question stem and hence had two equations with two unknowns.
For instance:
(I)x+y=15
(II)xy=9
If I substitute (II) into (I) I think I get the unique solution of y=3 and x=12?
Is it not allowed to combine an additional information from the question stem (here x+y=15) with each of the separate statements or where am I wrong?
I understand that if I'm looking at each statement, without considering x+y=15 from the question stem, then there are more than one possible solution. However, I included x+y=15 from the question stem and hence had two equations with two unknowns.
For instance:
(I)x+y=15
(II)xy=9
If I substitute (II) into (I) I think I get the unique solution of y=3 and x=12?
Is it not allowed to combine an additional information from the question stem (here x+y=15) with each of the separate statements or where am I wrong?
 GMATGuruNY
 GMAT Instructor
 Posts: 15537
 Joined: 25 May 2010
 Location: New York, NY
 Thanked: 13060 times
 Followed by:1906 members
 GMAT Score:790
Is 4^(x+y)=8^10?tobiasaebi wrote:Thank you for your answer, Neel.
I understand that if I'm looking at each statement, without considering x+y=15 from the question stem, then there are more than one possible solution. However, I included x+y=15 from the question stem and hence had two equations with two unknowns.
For instance:
(I)x+y=15
(II)xy=9
If I substitute (II) into (I) I think I get the unique solution of y=3 and x=12?
Is it not allowed to combine an additional information from the question stem (here x+y=15) with each of the separate statements or where am I wrong?
(2Â²)^(x+y) = (2Â³)Â¹â�°
2^(2x+2y) = 2Â³â�°
Since the bases are equal, so must be the exponents:
2x+2y = 30
x+y = 15.
Question rephrased: Is x+y = 15?
This is not INFORMATION but a QUESTION: IS x+y=15?
It is not KNOWN that x+y=15.
Quite the opposite: whether x+y=15 is what we are being ASKED to determine.
Since it is not KNOWN that x+y=15, we can't combine x+y=15 with the statements.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 testtakers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia  a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua  a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 testtakers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia  a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua  a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3

 Senior  Next Rank: 100 Posts
 Posts: 35
 Joined: 27 Jul 2011
 Followed by:2 members
tobiasaebi,tobiasaebi wrote:Hi all,
I was solving question #41 of the OG13 and don't understand the solutions...
The question asks:
Is 4^(x+y)=8^10?
(1) xy=9
(2) y/x=1/4
The answer is C
My solution was:
1. Restate the question into: 2^2(x+y)=2^30 which equals x+y=15
2. Looked at statement 1 which i restated into: x=9+y
3. I substituted step 2 into step 1 and got x=12 and y=3, therefore statement 1 should be sufficient?
4. I checked whether statment 2 is sufficient. Statement 2 was transformed to x=4y, which I substituted into the question and got 4y+y=15 equals y=3 and x=12. Therefore I thought that statement 2 is also sufficient. Thus I went for D.
Why is it not D?
Thank you for your answers!
Rephrasing the question:
Is 4^(x+y)=8^10 ?? or is 2^2(x+y)=2^3*10 ?? or is 2(x+y)=30 ??
(1) xy=9 Insufficient
since, if x=12 y=3, then YES it satisfies x+y=15.
if x=11 y=2, then No it doesn't satisfies x+y=15.
(2) y/x=1/4. or x=4y Also insufficient
Since, if x=12 y=3, then YES it satisfies x+y=15.
if x=4 y=1, then No it doesn't satisfies x+y=15.
And if we combine (1)&(2)
we two equations with 2 unknowns (xy=9 and y/x=1/4) hence we can solve x and y.
hence we get x=12 y=3, which satisfies x+y=15.