Find all School-related info fast with the new School-Specific MBA Forum

It is currently 27 Aug 2014, 13:06

Close

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
Your Progress

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

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

multiply numbers

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Senior Manager
Senior Manager
avatar
Joined: 10 Nov 2010
Posts: 268
Location: India
Concentration: Strategy, Operations
GMAT 1: 520 Q42 V19
GMAT 2: 540 Q44 V21
WE: Information Technology (Computer Software)
Followers: 5

Kudos [?]: 46 [0], given: 22

GMAT Tests User
multiply numbers [#permalink] New post 08 Mar 2011, 07:34
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

33% (02:30) correct 67% (00:47) wrong based on 6 sessions
15.) The product of all the prime numbers less than 20 is closest to which of the following powers of 10?

a) 10^9
b) 10^8
c) 10^7
d) 10^6
e) 10^5


2*3*5*7*11*13*17*19

10*21*143*17*19=

any easy way because i cnt do above multiplication in less time.

Thanks
[Reveal] Spoiler: OA

_________________

The proof of understanding is the ability to explain it.

Manager
Manager
avatar
Joined: 02 Apr 2010
Posts: 103
Followers: 5

Kudos [?]: 91 [0], given: 18

Re: multiply numbers [#permalink] New post 08 Mar 2011, 08:06
As you don't have to know the exact result but rather the magnitude I would just round when multiplying, i.e.:

2*3*5*7 = 210 (use 200)
200*11= 2200 (use 2,000)
2,000*13= 26000 (use 20,000)
20,000*17 = 340,000 (use 300,000)
300,000*19 = 5,700,000 (use 6,000,000)

Now since you rounded to a lower number most of the time and the result is still larger than 5,000,000 it is clear that the answer is C) 10^7.

I don't know why the answer is given as D) but I think it's wrong.
Manager
Manager
avatar
Joined: 14 Feb 2011
Posts: 198
Followers: 4

Kudos [?]: 59 [0], given: 3

GMAT Tests User
Re: multiply numbers [#permalink] New post 08 Mar 2011, 08:32
GMATD11 wrote:
15.) The product of all the prime numbers less than 20 is closest to which of the following powers of 10?

a) 10^9
b) 10^8
c) 10^7
d) 10^6
e) 10^5


2*3*5*7*11*13*17*19

10*21*143*17*19=

any easy way because i cnt do above multiplication in less time.

Thanks


Group these numbers so as to get product close to a multiple of 10, so they can be rewritten as (2*5)(3*7)(11*19)(13*17) or 10*21*209*221
Now take out power of 10 from each number and rewrite

10*10*100*100*(1*2.1*2.09*2.21) or 10^6*(1*2.1*2.09*2.21)

Which is greater than 10^6 and closer to 10^7, so answer should be C.

Are you sure OA is correct?
Director
Director
avatar
Status: Matriculating
Affiliations: Chicago Booth Class of 2015
Joined: 03 Feb 2011
Posts: 931
Followers: 11

Kudos [?]: 190 [0], given: 123

Reviews Badge
Re: multiply numbers [#permalink] New post 08 Mar 2011, 08:43
2*3*5*7*11*13*17*19

6 * 35 * 7 * 143 * 340 i.e.(17 * 20)
6 * 245 * 143 * 340
6 * 2.5 * 1.4 * 3.4 * 10^6
15 * 5 * 10^6
75 * 10^6
7.5 * 10^7
Re: multiply numbers   [#permalink] 08 Mar 2011, 08:43
    Similar topics Author Replies Last post
Similar
Topics:
Experts publish their posts in the topic A number a is squared and then multiplied by negative 3. The macjas 2 27 May 2012, 08:22
What is the least number that should be multiplied to 100! cool_jonny009 4 11 Nov 2005, 22:11
A certain set of numbers is formed by multiplying the first GMATT73 4 24 Sep 2005, 16:44
What is the least number that should be multiplied to 100! christoph 3 06 May 2005, 09:05
A boy multiplied 423 by a certain number and obtained 65589 anirban16 5 18 Mar 2005, 14:09
Display posts from previous: Sort by

multiply numbers

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

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®.