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Board of Directors P
Status: QA & VA Forum Moderator
Joined: 11 Jun 2011
Posts: 4486
Location: India
GPA: 3.5
WE: Business Development (Commercial Banking)
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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abhi758 wrote:
If x and y are positive integers and 450y=x^3, which of the following must be an integer?

i) $$\frac{y}{{3*2^2*5}}$$
ii) $$\frac{y}{{3^2*2*5}}$$
iii) $$\frac{y}{{3*2*5^2}}$$

a. None
b. i only
c. ii only
d. iii only
e. i, ii and iii

Please explain your answers..

$$450y=x^3$$

$$5^2*2*3^2*y = x^3$$

The minimum value of $$y =$$ $$2^2*5*3$$

Hence, among the given options, only (I) is possible...

So, correct answer must be (B)

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Abhishek....

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Current Student B
Joined: 20 Jan 2017
Posts: 58
Location: United States (NY)
Schools: CBS '20 (A)
GMAT 1: 750 Q48 V44 GMAT 2: 610 Q34 V41 GPA: 3.92
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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1)If cube root of 450y is an integer, then 450y has to have three of each prime factors. 2)Prime factors of 450 are 2*5*5*3*3. It is missing two 2's, one 5, and one 3. 3)To compensate for these missing factors y has to have all of them as its prime factors, consequently y has to be divisible by 2^2*5*3.
Current Student B
Joined: 20 Jan 2017
Posts: 58
Location: United States (NY)
Schools: CBS '20 (A)
GMAT 1: 750 Q48 V44 GMAT 2: 610 Q34 V41 GPA: 3.92
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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1) x is cubed, which means that its prime factors can be divided into three identical sets.
2) 450*y should have the same three sets of prime factors as x cubed, since the two expressions are equal
3) The prime factors of 450 are 2*5*5*3*3. This means that y has to consist of 2^2*5*3 at least, and it has to be divisible by this expression.

The correct answer is B.
Intern  Joined: 15 Feb 2017
Posts: 8
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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Cant we take higher values of N and Y so that it satisfies the equation,the all 3 possiblities are correct and answer is E
eg:
y= 5^4*3^4*2^5
N=5^2*3^2*2^2
Math Expert V
Joined: 02 Sep 2009
Posts: 55271
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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jakexix wrote:
Cant we take higher values of N and Y so that it satisfies the equation,the all 3 possiblities are correct and answer is E
eg:
y= 5^4*3^4*2^5
N=5^2*3^2*2^2

The question asks which of the following MUST be an integer, not COULD be an integer. If y = 2^2*3*5, which is the least possible value of y, then only B is true. Please re-read the thread and follow the links to similar questions to understand the concept better.
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GMAT 1: 630 Q48 V28 WE: Programming (Computer Software)
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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Bunuel wrote:
jakexix wrote:
Cant we take higher values of N and Y so that it satisfies the equation,the all 3 possiblities are correct and answer is E
eg:
y= 5^4*3^4*2^5
N=5^2*3^2*2^2

The question asks which of the following MUST be an integer, not COULD be an integer. If y = 2^2*3*5, which is the least possible value of y, then only B is true. Please re-read the thread and follow the links to similar questions to understand the concept better.

Hey Bunuel,

I have a query. Why are we considering only minimum value of y?

y = 2^8 * 3^7 * 5^7 also gives a integer value for n.
In that case, all the 3 options gives integers.

Please clarify. What am I missing here?
Senior Manager  P
Joined: 10 Apr 2018
Posts: 260
Location: United States (NC)
Re: If n and y are positive integers and 450y=n^3  [#permalink]

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loneRanger1050 wrote:
Bunuel wrote:
jakexix wrote:
Cant we take higher values of N and Y so that it satisfies the equation,the all 3 possiblities are correct and answer is E
eg:
y= 5^4*3^4*2^5
N=5^2*3^2*2^2

The question asks which of the following MUST be an integer, not COULD be an integer. If y = 2^2*3*5, which is the least possible value of y, then only B is true. Please re-read the thread and follow the links to similar questions to understand the concept better.

Hey Bunuel,

I have a query. Why are we considering only minimum value of y?

y = 2^8 * 3^7 * 5^7 also gives a integer value for n.
In that case, all the 3 options gives integers.

Please clarify. What am I missing here?

Hi loneRanger1050,

I understand as to what led you to think that all the three choices could suffice .

Let's see what led you to this conclusion.

if write n^3 in its prime factor form
$$n^{3}$$= $$5^{2}$$$$3^{2}$$ 2 y

Now since n is a integer , say n=k, then $$n^{3}$$= $$k^{3}$$, which means if we factorize n in its prime factors, then for $$n^{3}$$ the prime factorization should have powers which are multiples of 3 .

Then y can be of the form =$$2^{2}$$$$2^{P}$$ $$3^{1}$$$$3^{Q}$$$$5^{1}$$$$5^{R}$$ $$A^{Z}$$
where P, Q R Z must be multiple of 3 and A is a prime number ( other than 2,3,5)

But notice the question Stem. It asks for a scenario in which $$MUST BE$$ true. Had they asked for a scenario of CAN BE TRUE your logic would be correct.

Since we are asked for must be true scenario, then the condition must also exist for smallest possible value of y also . And least value of y can be $$2^{2}$$$$3^{1}$$$$5^{1}$$

Hope this helps
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Probus

~You Just Can't beat the person who never gives up~ Babe Ruth
Manager  B
Joined: 11 Jun 2018
Posts: 84
GMAT 1: 500 Q39 V21 Re: If n and y are positive integers and 450y=n^3  [#permalink]

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For those of you, who are more comfortable plugging in values, follow the below approach:-

450*Y=X^3

Now, simply think of numbers that are perfect cubes

we have 8,27,64 etc.

Now, try to get 2700, so we have

450*60=30^3

Perfect! Now Y=60, X=30

Test the answers now:-
1. We have 60/60 - an integer
2. 60/90-not an integer
3. 60/150-not an integer. Re: If n and y are positive integers and 450y=n^3   [#permalink] 15 Apr 2019, 08:23

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# If n and y are positive integers and 450y=n^3

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