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If y is the smallest positive integer such that 3,150 multip

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If y is the smallest positive integer such that 3,150 multip [#permalink]

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If y is the smallest positive integer such that 3,150 multiplied by y is the square of an integer, then y must be

(A) 2
(B) 5
(C) 6
(D) 7
(E) 14
[Reveal] Spoiler: OA

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Last edited by Bunuel on 17 Dec 2012, 08:53, edited 1 time in total.
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Re: y - the smallest +ve integer [#permalink]

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GMATD11 wrote:
142.) If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be

a) 2
b) 5
c) 6
d) 7
e) 14

upon factoring 3150 i got following prime factors 3,3,5,5 nd 2


3,150=2*3^2*5^2*7, now 3,150*y to be a perfect square y must complete the odd powers of 2 and 7 to even number (perfect square has even powers of its primes), so the least value of y is 2*7=14. In this case 3,150y=(2*3^2*5^2*7)*(2*7)=(2*3*5*7)^2=perfect square.

Answer: E.

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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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GMATD11 wrote:
If y is the smallest positive integer such that 3,150 multiplied by y is the square of an integer, then y must be

(A) 2
(B) 5
(C) 6
(D) 7
(E) 14


Solution:

This problem is testing us on the rule that when we express a perfect square by its unique prime factors, every prime factor's exponent is an even number.

Let’s start by prime factorizing 3,150.

3,150 = 315 x 10 = 5 x 63 x 10 = 5 x 7 x 3 x 3 x 5 x 2

3,150 = 2^1 x 3^2 x 5^2 x 7^1

(Notice that the exponents of both 2 and 7 are not even numbers. This tells us that 3,150 itself is not a perfect square.)

We also are given that 3,150 multiplied by y is the square of an integer. We can write this as:

2^1 x 3^2 x 5^2 x 7^1 x y = square of an integer

According to our rule, we need all unique prime factors' exponents to be even numbers. Thus, we need one more 2 and one more 7. Therefore, y = 7 x 2 = 14

Answer is E.
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Re: y - the smallest +ve integer [#permalink]

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GMATD11 wrote:
142.) If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be

a) 2
b) 5
c) 6
d) 7
e) 14

upon factoring 3150 i got following prime factors 3,3,5,5 nd 2


You missed 7 as a prime factor of 3150.

3150 = 3,3,5,5,2 and 7
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Re: If y is the smallest positive integer such that 3150 [#permalink]

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E.

Create a prime factor tree for 3,150 and see what numbers do not have a pair.
3150 = 5x5x3x3x7x2.... the 7 and 2 do not have a second pair to make it a perfect square of a number. so y must be 7*2 = 14
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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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hfbamafan wrote:
Where can I find the logic of how to answer this question?

Is there a section in the math guide that covers this?

Thanks,
Hunter


No special section covers this, but I can recommend similar questions:
if-m-and-n-are-positive-integer-and-1800m-n3-what-is-108985.html
property-of-integers-104272.html
if-x-and-y-are-positive-integers-and-180x-y-100413.html
number-properties-92562.html
can-someone-answer-this-and-tell-me-why-92066.html
og-quantitative-91750.html
division-factor-88388.html
if-5400mn-k4-where-m-n-and-k-are-positive-integers-109284.html

Hope it helps.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: y - the smallest +ve integer [#permalink]

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New post 09 Mar 2011, 17:28
Answer is E.

on prime factorization 3150 can be expressed as 2 *(3^2)*(5^2)*7

to make the above number a perfect square , we know it atleast need to multiply this by 2*7 = 14

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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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New post 17 Jul 2013, 21:09
Where can I find the logic of how to answer this question?

Is there a section in the math guide that covers this?

Thanks,
Hunter
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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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New post 11 May 2016, 00:44
3150 = 3 * 1050 = 3*3*350 = 3*3*35*10
= 3*3*5*7*5*2
= (3)^2 * (5)^2 * 14
So we can say
if we want this number to be a perfect square
this has to be multiplied with 14 .
y=14
:)

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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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New post 11 May 2016, 02:02
Logic for a number to be square = all its prime factors should have power 2

3150 = (5^2)(3^2)(2*7)

Only 2 and 7 do not have power 2

to make power 2 multiply with 2*7=14

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Re: If y is the smallest positive integer such that 3,150 multip [#permalink]

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Re: If y is the smallest positive integer such that 3,150 multip   [#permalink] 15 May 2017, 10:37
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