If N is a positive integer...

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

If N is a positive integer...

by BTGmoderatorLU » Tue Oct 17, 2017 6:46 pm

If N is a positive integer and 14N/60 is an integer. What is the smallest value of N for which N has exactly four different prime factors?

A)30
B)60
C)180
D)210
E)cannot be determined

The OA is D.

Why D is the correct answer? Can any expert explain this PS question please?. Thanks.

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Source: Beat The GMAT — Problem Solving | Tue Oct 17, 2017 6:46 pm

EconomistGMATTutor: GMAT Instructor; Posts: 555; Joined: Wed Oct 04, 2017 4:18 pm; Thanked: 180 times; Followed by:12 members

by EconomistGMATTutor » Tue Oct 17, 2017 7:54 pm

Hello.

If you see carefully the options on the list you can determine which is the correct answer.

Let's see option by option.

- Option A: 30
30=2*3*5, so 30 has 3 different prime factors. OUT

- Option B: 60
60=2*2*3*5, so 60 has 3 different prime factors. OUT.

- Option C: 180
180=2*2*3*3*5, so 180 has 3 different prime factors, OUT.

- Option D: 210
210=2*3*5*7, so 210 has exactly 4 different prime factors. In addition, 14*210/60=49, so 14N/60 is an integer. CORRECT.

We don't need to analyze the last option.

So, the answer is D.

I hope this answer could help you.

I'm available if youd like any follow up.

GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pm; Location: New York, NY; Thanked: 13060 times; Followed by:1906 members; GMAT Score:790

by GMATGuruNY » Wed Oct 18, 2017 2:39 am

LUANDATO wrote:If N is a positive integer and 14N/60 is an integer. What is the smallest value of N for which N has exactly four different prime factors?

A)30
B)60
C)180
D)210
E)cannot be determined

14N/60 = 7N/30.

The smallest prime numbers are 2, 3, 5 and 7.
Thus, the smallest integer with 4 different prime factors = 2*3*5*7 = 210.
Test whether N=210 yields an integer value for 7N/30:
(7*210)/30= 7*7 = 49.
Success!
Thus, the least possible value for N = 210.

The correct answer is D.

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

by BTGmoderatorLU » Wed Oct 18, 2017 9:27 am

Hello,

Excellent, your explanations was helpful to me a lot! Thank you so much!

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Sun Oct 22, 2017 3:43 pm

LUANDATO wrote:If N is a positive integer and 14N/60 is an integer. What is the smallest value of N for which N has exactly four different prime factors?

A)30
B)60
C)180
D)210
E)cannot be determined .

We are given that 14N/60 is an integer, and we need to determine the smallest value of N for which N has exactly four different prime factors.

14N/60 = 7N/30

In order for 7N/30 to be an integer, N must be divisible by 30. In other words, N must be a multiple of 30.

However, since 30 = 2 x 3 x 5, which are 3 different primes, the smallest value of n with 4 different primes is 2 x 3 x 5 x 7 = 210.

Answer: D

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