It is currently 22 Feb 2018, 18:41

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.

Close

Request Expert Reply

Confirm Cancel

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

PRODUCT OF PRIME NUMBER

  post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
Intern
Intern
avatar
Joined: 01 May 2009
Posts: 20
PRODUCT OF PRIME NUMBER [#permalink]

Show Tags

New post 02 Aug 2009, 05:36
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

HideShow timer Statistics

This topic is locked. If you want to discuss this question please re-post it in the respective forum.

The product of all prime number less than 20 is closest to which of the following power of 10?
a)10^9
b) 10^8
c) 10^7
d) 10^6
e) 10^5

is there any quick way to slove it?

Thanks
1 KUDOS received
Manager
Manager
avatar
Joined: 10 Jul 2009
Posts: 163
Re: PRODUCT OF PRIME NUMBER [#permalink]

Show Tags

New post 02 Aug 2009, 07:22
1
This post received
KUDOS
2,3,5,7,11,13,17,19
Mutlitply terms to for products nearest to multiples of 10.
10,21,143,17,19

10^3*4*143*17 ~ 10^3 * 150 * 4 *20 ~10^7

So answer is C.
2 KUDOS received
Intern
Intern
avatar
Joined: 02 Aug 2009
Posts: 3
Schools: McCombs '17
Re: PRODUCT OF PRIME NUMBER [#permalink]

Show Tags

New post 02 Aug 2009, 07:30
2
This post received
KUDOS
Here is another method to work your way to the answer.

After making a list of prime numbers, I would look for ways to make multiplication easier.

For instance, 19 and 11 are both prime numbers under twenty. If I took the time to times them, my answer would be 209. A quicker way to get an estimate is to shift '1' from '11' to '19'. In effect, making '11' become '10' and making '19' become '20'. Multiplying 10 and 20 gives me 200. A fairly close estimate.

Then, with 7 and 13, I could shift '3' from '13' to '7' making two '10.' 7 times 13 is 91. 10 times 10 is 100. Another close estimate.

Following those we could do 2 x 3 x 5 easily enough in our heads, giving us 30 and leaving us with 17 unmultiplied.

Our new formula is 200 x 100 x 30 x 17. Counting ones and tens places we get '6.' Making our starting estimate 10^6, which we know is 1,000,000. Times 1,000,000 by 2 then 3 then 1.7, which gives us an estimate of 10,200,000. 10^7 is therefore our answer. Either 1.5 or 2 could be used in the place of 1.7 for quicker multiplication.

Had we multiplied numbers individually our answer would have been 9699690. Just under 10^7. This method isn't precise but, when choosing between widely spread answer choices, it normally works.
Re: PRODUCT OF PRIME NUMBER   [#permalink] 02 Aug 2009, 07:30
Display posts from previous: Sort by

PRODUCT OF PRIME NUMBER

  post reply Question banks Downloads My Bookmarks Reviews Important topics  


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

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®.