Quant - Official GMAC Software Practice Test Questions - The Beat The GMAT Forum

Quant - Official GMAC Software Practice Test Questions

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skalevar: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Wed Feb 03, 2010 5:21 pm; GMAT Score:720

by skalevar » Sun Jul 18, 2010 1:12 pm

I'd love some additional explanation regarding the answer to the questions below, from the software simulated Practice Test available from mba.com

If n is a positive integer less than 200 and 14n/60 is an integer, then n has how many different prime factors?

A. 2
B. 3
C. 5
D. 6
E. 8

I'm really kind of stuck on this questions, so if you have any insights, they would be much appreciated!

Thanks,

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Source: Beat The GMAT — Quantitative Reasoning | Sun Jul 18, 2010 1:12 pm

debmalya_dutta: Master | Next Rank: 500 Posts; Posts: 385; Joined: Sun Jul 12, 2009 10:16 pmFollowed by: 2 members; Thanked: 29 times; GMAT Score:710

by debmalya_dutta » Sun Jul 18, 2010 1:47 pm

The answer is 3 I think.
Here's how I did it ...
n<200 and 14n/60 is an integer

(2* 7 * n)/ 2* 2* 3 * 5
= (7 * n)/ 2* 3 * 5

Now , for [(7 * n)/ 2* 3 * 5] to be an integer , n should be a multiple of 30 (denominator = 30)
number of Prime factors of 30 = 3

Note that since n< 200 , we cannot consider the next multiple of 30 which will introduce the another prime number which would be 7 because then n will become 210 and greater than 200

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paruloberai: Junior | Next Rank: 30 Posts; Posts: 11; Joined: Sun May 17, 2009 7:04 am; Location: India

by paruloberai » Tue Jul 20, 2010 11:25 pm

skalevar wrote:I'd love some additional explanation regarding the answer to the questions below, from the software simulated Practice Test available from mba.com

If n is a positive integer less than 200 and 14n/60 is an integer, then n has how many different prime factors?
A. 2
B. 3
C. 5
D. 6
E. 8
I'm really kind of stuck on this questions, so if you have any insights, they would be much appreciated!
Thanks,

Hi skalevar,

The easiest way to approach this question is to a pick number. Since, the answer has to be a unique value this means that for any value of n ( n should be positive and less than 200), the number of different prime factors of n is same. Also, note that for this value of n, 14n/60 should be an integer. The easiest number to pick is n=60, then 14n/60= 14. Now, see 60= 2*2*3*5. Hence, 3 different prime factors. Answer B.

Parul Oberai is a content expert for GMATLounge. She has a lot of experience helping GMAT students to be more efficient in solving quantitative problems. She can be reached at https://gmatlounge.com/ .

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sumanr84: Legendary Member; Posts: 758; Joined: Sat Aug 29, 2009 9:32 pmFollowed by: 2 members; Location: Bangalore,India; Thanked: 67 times

by sumanr84 » Wed Jul 21, 2010 7:42 am

I would prefer debmalya_dutta's way of using Prime Factorization..its very easy..

I am on a break !!

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outreach: Legendary Member; Posts: 748; Joined: Sun Jan 31, 2010 7:54 amFollowed by: 3 members; Thanked: 46 times

by outreach » Sun Jul 25, 2010 11:44 pm

(14n)/60=(7n)/30

n should be divisible by 30

n=30,60,90.....180

each has 3 diff prime factors

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