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Target Test Prep Representative D
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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?

(1) x - y = 9
(2) y/x = 1/4

We need to determine whether 4^(x+y) = 8^10. We start by breaking down our two bases into prime factors.

4^(x+y) = (2^2)^(x+y) = 2^(2x+2y)

8^10 = (2^3)^10 = 2^30

We can now rephrase the question as:

Is 2^(2x+2y) = 2^30 ?

Because the bases are the same, we can drop them and set the exponents equal to each other. The question becomes:

Is 2x+2y = 30 ?

Is x + y = 15 ?

After simplifying the equation, we see that we need to determine whether the sum of x and y is equal to 15.

Statement One Alone:

x – y = 9

Knowing the difference of x and y is not the same as knowing the sum of x and y; thus, statement one is not sufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

y/x = ¼

When we cross multiply obtain:

4y = x

4y = x is not enough information to determine the value of x + y. Statement two alone is not sufficient. We can eliminate answer choice B.

Statements One and Two Together:

Using statements one and two we know the following:

x – y = 9 and 4y = x

Since 4y = x, we can substitute 4y for x into the equation x – y = 9 and we have:

4y – y = 9

3y = 9

y = 3

Since y = 3, x = 4(3) = 12.

Thus, x + y = 12 + 3 = 15. We can answer yes to the question. Both statements together are sufficient.

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Is $$4^{x+y}=8^{10}$$ ?

$$4^{x+y}=8^{10}$$

$$2^{2(x+y)}=2^{30}$$

$$2(x+y) = 30$$

$$x+y = 15$$

So, we will find if $$x+y = 15$$?

(1) $$x - y = 9$$

Clearly not sufficient as we can have multiple values for x and y

Hence, (1) ===== is NOT SUFFICIENT

(2) \frac{y}{x = 1/4}

$$x = 4y$$

Again this is not sufficient as we can get multiple values of x and y

Hence, (2) ===== is NOT SUFFICIENT

Combining (1) & (2)

When we substitute $$x = 4y$$ in equation $$x - y = 9$$

we get,

$$4y - y = 9$$

$$3y = 9$$

$$y = 3$$

$$x = 4y$$

$$x = 4 * 3$$

$$x = 12$$

$$x + y = 12 + 3 = 15$$

Hence, (1) & (2) ===== is SUFFICIENT

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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?

(1) x - y = 9
(2) y/x = 1/4

Target question: Is 4^(x + y) = 8^10?
This is a good candidate for rephrasing the target question.

Given equation: 4^(x + y) = 8^10
Rewrite 4 and 8 as powers of 2 to get: (2²)^(x + y) = (2³)^10
Apply power of a power law to get: 2^(2x +2y) = 2^30
This means that: 2x + 2y = 30
Divide both sides by 2 to get: x + y = 15
In other words, asking whether 4^(x + y) = 8^10 is the SAME as asking whether x + y = 15
REPHRASED target question: Is x + y = 15?

Statement 1: x - y = 9
Is this enough information to answer the REPHRASED target question? No.
Consider these two CONFLICTING cases:
Case a: x = 12 and y = 3. In this case, x + y = 12 + 3 = 15. So, x + y DOES equal 15
Case b: x = 10 and y = 1. In this case, x + y = 10 + 1 = 11. So, x + y does NOT equal 15
Since we cannot answer the REPHRASED target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: y/x = 1/4
Is this enough information to answer the REPHRASED target question? No.
Consider these two CONFLICTING cases:
Case a: x = 12 and y = 3. In this case, x + y = 12 + 3 = 15. So, x + y DOES equal 15
Case b: x = 8 and y = 2. In this case, x + y = 8 + 2 = 10. So, x + y does NOT equal 15
Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Statement 1 tells us that x - y = 9
Statement 2 tells us that y/x = 1/4
Since we have 2 different linear equations with 2 variables, we COULD solve the system for the individual values of x and y, which means we COULD answer the REPHRASED target question with certainty. Of course, we wouldn't waste precious time performing such calculations, since our sole goal is to determine the sufficiency of the combined statements.
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

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Originally posted by GMATPrepNow on 03 Aug 2017, 14:58.
Last edited by GMATPrepNow on 15 Jan 2018, 09:00, edited 1 time in total.
Senior Manager  G
Joined: 06 Jul 2016
Posts: 360
Location: Singapore
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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?
(1) x - y = 9
(2) y/x = 1/4

Q Stem ->$$2^2(x+y)$$ = $$2^30$$
=> x+y = 15 ?

1) x - y = 9
x = 10, y = 1 => x + y = 11 => NO
x = 12, y = 3 => x + y = 15 => YES
Insufficient.

2) x = 4y
x = 4, y = 1 => x + y = 5 => NO
x = 12, y = 3 => x + y = 15 => YES
Insufficient.

1+2)
4y - y = 9
=> y = 3 => x = 12
Sufficient.

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Intern  B
Joined: 04 Mar 2017
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I understand that statement one alone is not sufficient, however when we plug in for statement 2 we end up with 4y=x. So wouldn’t it be as follows,
4y+y= 15
5y= 15
Y= 3 and then we can solve for x
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Ekjotb wrote:
I understand that statement one alone is not sufficient, however when we plug in for statement 2 we end up with 4y=x. So wouldn’t it be as follows,
4y+y= 15
5y= 15
Y= 3 and then we can solve for x

Hello

It is NOT given that 4^(x+y) = 8^10. We have to determine whether this is actually true or not.

You are assuming that 4^(x+y) equals 8^10.
Intern  B
Joined: 10 Jun 2018
Posts: 10

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Hey Guys,

Quick question from my side. Why are we allowed in Statement B to make this move:

y/x=1/4 -> 4y = x

I am confused, because we dont know for sure that either x or y are really positive and hence we would not be allowed to divide or multiple a variable.

Math Expert V
Joined: 02 Sep 2009
Posts: 56304

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1
HHPreparation wrote:
Hey Guys,

Quick question from my side. Why are we allowed in Statement B to make this move:

y/x=1/4 -> 4y = x

I am confused, because we dont know for sure that either x or y are really positive and hence we would not be allowed to divide or multiple a variable.

We are concerned about the sign when dealing with inequalities: we should keep the sign if we multiply by a positive value and flip the sign when we multiply by a negative value.

For equations we can multiply/divide by a variable regardless of its sign (providing it's not 0).

Hope it's clear.
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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?

(1) x - y = 9
(2) y/x = 1/4

Practice Questions
Question: 41
Page: 278
Difficulty: 600

Basically the question is asking if x + y = 15

From statement 1) we don’t have enough information so not sufficient.

Now for 2) y/x = 1/4 we know x = 4y

We don’t have enough information.

So now if we combine both

We get x - y = 9 and x = 4y

4 - y = 9, we get y = 3 and x = 12

x + y = 15

C is sufficient.

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Intern  B
Affiliations: National Institute of Technology, Durgapur
Joined: 22 Feb 2017
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GMAT 1: 720 Q49 V38 GPA: 3.6
WE: Engineering (Manufacturing)

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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?

(1) x - y = 9
(2) y/x = 1/4

Practice Questions
Question: 41
Page: 278
Difficulty: 600

From the question, 2^2(x+y) = 2^3x10 =>v2x + 2y = 30 => x+y = 15 ?? That's what it asks basically ;

Now, from (1), x-y=9 ; We can say there are 2 variables & 1 equation so there are many values possible for which x-y = 9
But, only the value 12-3=9 & 12+3=15 for all other values we can say whatever maybe the value of x or y such that x-y=9 it won't be equal to 15 ;
Since, we don't know what are their values so (1) is clearly INSUFFICIENT.

From, (2), 4y=x ; Same here we can't say what are the value of x & y ;
But, here only & only if x=3 & y=4x3=12 then only we can get 12+3=15 ;
But, we are not sure as per (2) & hence, (2) is INSUFFICIENT.

From, (1) & (2),
we can determine x & y and after solving we get x=12 & y=3 => x+y = 15 => That's what the question asks => SUFFICIENT => Answer option C ;
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Joined: 09 Mar 2018
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Bunuel wrote:
Is $$4^{x+y}=8^{10}$$ ?

(1) x - y = 9
(2) y/x = 1/4

Lets just manipulate the original to get

$$2^{2x+2y}=2^{30}$$
2(x+y) = 30
x +y = 15

Lets forget about it for now

Lets attack 2 first, its easy to get values from it
x = 4y
y = 1,2,3
x = 4,8,12

12+3 = 15, yes to the question
4+1 != 15, No to the question

from 1) from 2 i got the unique value which can satisfy statement 1 as well

x - y = 9,
12-3 = 9 , 12+3 = 15, yes to the question
10-1 = 9, 10+1 != 15, No to the question

On only combining both the statements we get the value as 12 & 3
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Many of life's failures happen with people who do not realize how close they were to success when they gave up. Re: Is 4^(x+y)=8^(10) ?   [#permalink] 18 Feb 2019, 00:01

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