It is currently 25 Feb 2018, 13:56

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# How many prime numbers between 1 and 100 are factors of 7150

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:

### Hide Tags

Director
Joined: 29 Aug 2005
Posts: 854
How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

13 Apr 2009, 00:39
8
KUDOS
8
This post was
BOOKMARKED
00:00

Difficulty:

35% (medium)

Question Stats:

62% (00:50) correct 38% (00:54) wrong based on 1561 sessions

### HideShow timer Statistics

How many prime numbers between 1 and 100 are factors of 7,150?

A. One
B. Two
C. Three
D. Four
E. Five
[Reveal] Spoiler: OA
Intern
Joined: 25 Dec 2008
Posts: 18
Schools: HBS, Stanford
Re: How many prime numbers between 1 and 100? [#permalink]

### Show Tags

13 Apr 2009, 03:05
7,150 = 2 * 5 * 5 * 11 * 13

so there are 4 distinct prime numbers that are below 100, so answer D.
Intern
Joined: 04 Jan 2010
Posts: 6
Re: How many prime numbers between 1 and 100? [#permalink]

### Show Tags

16 Jan 2012, 11:09
2
KUDOS
Hate to play devil's advocate here, but the question doesn't clarify how many UNIQUE prime numbers are factors of 7150. (Which would change the answer to five). Am I missing something?
Math Expert
Joined: 02 Sep 2009
Posts: 43916
Re: How many prime numbers between 1 and 100? [#permalink]

### Show Tags

16 Jan 2012, 11:33
1
KUDOS
Expert's post
3
This post was
BOOKMARKED
jgonza8 wrote:
Hate to play devil's advocate here, but the question doesn't clarify how many UNIQUE prime numbers are factors of 7150. (Which would change the answer to five). Am I missing something?

How many prime numbers between 1 and 100 are factors of 7,150?
A. One
B. Two
C. Three
D. Four
E. Five

Make prime factorization of 7,150 --> 7,150=2*5^2*11*13. So 4 prime numbers between 1 and 100 (namely 2, 5, 11, and 13) are factors of 7,150 (you shouldn't count one prime factor twice).

_________________
Manager
Status: MBA Aspirant
Joined: 12 Jun 2010
Posts: 169
Location: India
Concentration: Finance, International Business
WE: Information Technology (Investment Banking)
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

17 Jan 2012, 03:17
Ans is D = 7150 = 2*5*5*11*13
so the prime numbers are 2,5,11,13
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7960
Location: Pune, India
Re: How many prime numbers between 1 and 100? [#permalink]

### Show Tags

18 Jan 2012, 04:24
2
KUDOS
Expert's post
jgonza8 wrote:
Hate to play devil's advocate here, but the question doesn't clarify how many UNIQUE prime numbers are factors of 7150. (Which would change the answer to five). Am I missing something?

Yes, you are! Read the question again:
How many prime numbers between 1 and 100 are factors of 7,150?

The prime numbers between 1 and 100 are 2, 3, 5, 7, 11, 13, 17, 19... etc

5 appears only once between 1 and 100 so there is absolutely no confusion. Out of these 25 prime numbers, only 4 are factors of 7150: 2, 5, 11 and 13
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Intern Joined: 14 Nov 2011 Posts: 11 Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink] ### Show Tags 13 Feb 2012, 18:09 3 This post received KUDOS I also got 2,5,7,11 and 13 but it took me more than the two minutes to break down 143 into 11 and 13. 2,5 and 7 were the easier ones. I'm sure many will agree at in times of stress, the mind starts to play tricks. At first I stopped after 7 and thought that 143 cannot be factored further and chose C as the answer i.e. 3 prime factors but then got the other two in over the two mins. Therefore, is there a quick way to get the prime factors of a large number? Thanks. Math Expert Joined: 02 Sep 2009 Posts: 43916 Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink] ### Show Tags 13 Feb 2012, 18:22 9 This post received KUDOS Expert's post 1 This post was BOOKMARKED pratk wrote: I also got 2,5,7,11 and 13 but it took me more than the two minutes to break down 143 into 11 and 13. 2,5 and 7 were the easier ones. I'm sure many will agree at in times of stress, the mind starts to play tricks. At first I stopped after 7 and thought that 143 cannot be factored further and chose C as the answer i.e. 3 prime factors but then got the other two in over the two mins. Therefore, is there a quick way to get the prime factors of a large number? Thanks. Generally there is no easy way to check whether some very large number is a prime (well if it doesn't have some small primes, which are easy to check). You'll need a computer to do this. Next, the GMAT won't give you a large number to factorize if there is no shortcut for that. For example in our original question after you find 2, 5, and 7, just check for the next prime 11: 143/11=13. You can also use divisibility rule for 11: if you sum every second digit and then subtract all other digits and the answer is divisible by 11, then the number is divisible by 11. So, for 143: (1+3)-4=0 --> 0 is divisible by 11 thus 143 is divisible by 11. Or, you can notice that 143=130+13=13*10+13, so 143 must be divisible by 13. So, as you can see there are plenty of shortcuts to get prime factorization of the numbers from the GMAT problems. For more on divisibility rules and on verifying the primality check Number Theory chapter of Math Book: math-number-theory-88376.html Hope it helps. _________________ Intern Joined: 16 Dec 2011 Posts: 46 GMAT Date: 04-23-2012 Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink] ### Show Tags 13 Feb 2012, 21:09 Good explanaation bunel thanks Posted from my mobile device Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7960 Location: Pune, India Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink] ### Show Tags 13 Feb 2012, 23:19 7 This post received KUDOS Expert's post 1 This post was BOOKMARKED pratk wrote: I also got 2,5,7,11 and 13 but it took me more than the two minutes to break down 143 into 11 and 13. 2,5 and 7 were the easier ones. I'm sure many will agree at in times of stress, the mind starts to play tricks. At first I stopped after 7 and thought that 143 cannot be factored further and chose C as the answer i.e. 3 prime factors but then got the other two in over the two mins. Therefore, is there a quick way to get the prime factors of a large number? Thanks. Sadly, you will need to check for all prime numbers till the square root of the given number. We don't know the square root of 143 but we can approximate it. 143 is very close to 144. The square root of 144 is 12 so you will need to check for all prime numbers less than 12. If none of the prime numbers less than 12 is a factor of 143, then you can say that 143 is prime. So the question is not over till you don't check for 11 too. So basically, you need to check for all prime factors less than $$\sqrt{n}$$ to figure out whether n is prime. If you are not sure why, check out these posts for details on factors: http://www.veritasprep.com/blog/2010/12 ... ly-number/ http://www.veritasprep.com/blog/2010/12 ... t-squares/ _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

Veritas Prep Reviews

Intern
Joined: 14 Nov 2011
Posts: 11
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

14 Feb 2012, 19:16
Thanks for the explanation and the additional posts Bunuel and Karishma..
Manager
Joined: 13 Feb 2012
Posts: 143
Location: Italy
Concentration: General Management, Entrepreneurship
GMAT 1: 560 Q36 V34
GPA: 3.1
WE: Sales (Transportation)
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

24 Jul 2012, 09:25
1
KUDOS
Had the same doubt about the "unique prime factors thing"; thanks for the clarification.
_________________

"The Burnout" - My Debrief

Kudos if I helped you

Andy

Manager
Joined: 23 May 2013
Posts: 189
Location: United States
Concentration: Technology, Healthcare
Schools: Stanford '19 (M)
GMAT 1: 760 Q49 V45
GPA: 3.5
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

28 Feb 2014, 05:43
I got the factorization in a much simpler way:

Multiples of 5 are extremely simple to factor; just multiply the number by 2 and divide by 10. In this case, I knew that 715 was a multiple of 5, so I knew 10*715 could be broken down into 10*(143*5). From there, I recognized 143 as a number that appears frequently on the gmat, and tested multiples of 11 to find 11 & 13. Then, the breakdown is simple: 2*5*5*11*13 = 4 distinct primes.

This took me 32 seconds to solve, so there are definitely simple methods to this type of problem.
SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1839
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

27 Mar 2014, 20:42
7150 is divisible by 11(7+5-1 = 11) & 25 (last two digits 50)

Remains 26 which is divisible by 13

7150 = 11 * 25 * 13 * 2

2, 5, 11, 13 are the prime factors

Answer = 4 = D
_________________

Kindly press "+1 Kudos" to appreciate

Senior Manager
Status: Math is psycho-logical
Joined: 07 Apr 2014
Posts: 432
Location: Netherlands
GMAT Date: 02-11-2015
WE: Psychology and Counseling (Other)
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

19 Jan 2015, 09:20
So, just to clarify this, if the question only asks for the factors we count all of the factors, no matter if there are repetitions.

Only when it specifically says that we need the "unique" factors should we diregard repeating factors.

Right?
SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1839
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

19 Jan 2015, 18:04
1
This post was
BOOKMARKED
pacifist85 wrote:
So, just to clarify this, if the question only asks for the factors we count all of the factors, no matter if there are repetitions.

Only when it specifically says that we need the "unique" factors should we diregard repeating factors.

Right?

That's correct. In this question, they have specifically mentioned prime numbers (which in turn are unique)
_________________

Kindly press "+1 Kudos" to appreciate

Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7960
Location: Pune, India
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

19 Jan 2015, 21:14
pacifist85 wrote:
So, just to clarify this, if the question only asks for the factors we count all of the factors, no matter if there are repetitions.

Only when it specifically says that we need the "unique" factors should we diregard repeating factors.

Right?

Not really.
Take the example of factors of 8:

How many total factors does 8 have and what are they?
They are 1, 2, 4 and 8 - a total of 4 factors

We know that 8 = 2^3 but we don't say that factors are 8 are 1, 2, 2, 2, 4 and 8.

Similarly, if we are asked - how many prime factors does 8 have? I will answer only 1 (the prime factor is 2). The number of prime factors of 8 are not 3 (not 2, 2, 2). I know of people who are not very convinced with this and hence, I assume that GMAT will insert the word "unique" to remove all doubts.

On GMAT, I would expect it to be - How many unique prime factors does 8 have?

In the original question, there is no doubt since they ask "how many prime numbers are factors of..." The set of prime numbers does not have multiple entries and hence there is no doubt that we are talking about unique prime factors only.
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for \$199

Veritas Prep Reviews

Director
Joined: 02 Sep 2016
Posts: 786
Re: How many prime numbers between 1 and 100 are factors of 7150 [#permalink]

### Show Tags

01 Apr 2017, 04:55
Factorize 7150

=2*5^2*11*13

Thus four prime factors.
_________________

Help me make my explanation better by providing a logical feedback.

If you liked the post, HIT KUDOS !!

Don't quit.............Do it.

Re: How many prime numbers between 1 and 100 are factors of 7150   [#permalink] 01 Apr 2017, 04:55
Display posts from previous: Sort by

# How many prime numbers between 1 and 100 are factors of 7150

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.