Divisibility and primes - Expert opinion - The Beat The GMAT Forum

Divisibility and primes - Expert opinion

Expert replies

[email protected]: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Mon Nov 17, 2014 4:45 am

by [email protected] » Sun May 24, 2015 7:47 am

I'm really struggling on the divisibility and primes questions. Specifically, the wording of the questions. I fully understand how to factor, primes etc, and I understand the Manhattan GMAT chapter on it and did well on the practice questions, but when I get to the OG I can't wrap my head around the wording. A lot of the time I don't understand what they are asking and spend so much time trying to figure it out. For example question 127, 155

Needing expert help on how I can better understand (simple technique) and translate the question, as well as and any additional sources online that I can practice doing more questions.

Thank-you!
B

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Source: Beat The GMAT — Problem Solving | Sun May 24, 2015 7:47 am

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Sun May 24, 2015 8:59 am

Hi Beth,

I'd like to know more about the specific issue/problem that you're facing. Can you post any of the prompts that you're referring to AND go into more detail about where you get 'stuck' in the question?

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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[email protected]: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Mon Nov 17, 2014 4:45 am

by [email protected] » Sun May 24, 2015 9:37 am

Hey Rich,
The issue is when I try read the question. I get confused at what they are asking, for example; If 3<x<100, for how many values of x is x/3 the square of a prime?

I don't know how to understand this, and quickly. I keep getting stuck on the wording and I am confused at what is being asked.

Or

If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be?

I read, reread and I am not clear on what the question is asking to begin with. This happens for multiple divisibility and prime questions. I keep getting frustrated at not being able to understand the prompt. I look at the answer and I get how they arrived at the numbers, but it's not clear on what the question was asking. Does that make sense? I may not be doing the best at explaining the issue...let me know if you need more examples.

Thanks for your help!

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Sun May 24, 2015 10:25 am

[email protected] wrote: If 3<x<100, for how many values of x is x/3 the square of a prime?

Here's one approach: https://www.beatthegmat.com/og-127-t276321.html

[email protected] wrote: If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be?

Several solutions here: https://qa.www.beatthegmat.com/factor-qu ... 70495.html

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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theCEO: Master | Next Rank: 500 Posts; Posts: 363; Joined: Sun Oct 17, 2010 3:24 pmFollowed by: 3 members; Thanked: 115 times

by theCEO » Sun May 24, 2015 10:35 am

Hi Beth,

At times I fall into the same spot. Here is what works for me. Break the problem up into smaller pieces as oppose to trying to do the problem as one whole for example

If 3<x<100, for how many values of x is x/3 the square of a prime?
I would start with x/3 the square of a prime
x/3 = (prime)^2
x = 3 (prime)^2

Then reanalyze the question
When is 3 < 3 (prime)^2 < 100 or
when is 1 < (prime)^2 < 33.3

This occurs when prime = 2,3,5, hence answer = 3

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Sun May 24, 2015 7:43 pm

Hi Beth,

Every question that you face on the GMAT can be broken down into pieces (much in the way that theCEO described). How YOU choose to break them down is up to you, but it helps to take notes and stay organized.

Here's how I'd approach your first example:

If 3<x<100, for how many values of x is x/3 the square of a prime?

Primes: 2, 3, 5, 7, 11, etc.
(Primes)^2: 4, 9, 25, 49, 121, etc.

X/3 = square of a prime, so X would equal...

4 --> X=12
9 --> X = 27
25 --> X = 75
49 --> X = 147 TOO BIG

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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nikhilgmat31: Legendary Member; Posts: 518; Joined: Tue May 12, 2015 8:25 pm; Thanked: 10 times

by nikhilgmat31 » Mon May 25, 2015 3:15 am

If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be?

First We need to find in factors of = sqrt ( 3150 *y) = sqrt(3*7*3*5*5*2*y)
to be able to find a perfect square root - y must be 7*2= 14

Ans: 14

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theCEO: Master | Next Rank: 500 Posts; Posts: 363; Joined: Sun Oct 17, 2010 3:24 pmFollowed by: 3 members; Thanked: 115 times

by theCEO » Mon May 25, 2015 5:14 am

nikhilgmat31 wrote:If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be?

First We need to find in factors of = sqrt ( 3150 *y) = sqrt(3*7*3*5*5*2*y)
to be able to find a perfect square root - y must be 7*2= 14

Ans: 14

Eventhough your answer is correct, the question asked for "square" and not "square root". Did you purposely meant to use "sqrt" in your solution?

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Mon May 25, 2015 6:44 am

[email protected] wrote: If y is the smallest positive integer such that 3150 multiplied by y is the square of an integer, then y must be?

Here's a similar question to practice with: https://www.beatthegmat.com/which-of-the ... 83100.html

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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nikhilgmat31: Legendary Member; Posts: 518; Joined: Tue May 12, 2015 8:25 pm; Thanked: 10 times

by nikhilgmat31 » Mon May 25, 2015 8:03 pm

answer asked for square of y so we did sqrt of 3150, Find prime factors of 3150 & make pairs.

integers not able to make pairs will be value of y.

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