GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 26 May 2019, 20:19

### 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

# The product of all the prime numbers less than 20 is closest to which

 post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
Manager
Joined: 10 Nov 2010
Posts: 178
Location: India
Concentration: Strategy, Operations
GMAT 1: 520 Q42 V19
GMAT 2: 540 Q44 V21
WE: Information Technology (Computer Software)
The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

08 Mar 2011, 08:34
1
1
00:00

Difficulty:

75% (hard)

Question Stats:

36% (01:01) correct 64% (00:55) wrong based on 73 sessions

### HideShow timer Statistics

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

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/the-product-o ... 35192.html

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________
The proof of understanding is the ability to explain it.
Manager
Joined: 02 Apr 2010
Posts: 99
Re: The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

08 Mar 2011, 09:06
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
Joined: 14 Feb 2011
Posts: 169
Re: The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

08 Mar 2011, 09:32
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
Status: Impossible is not a fact. It's an opinion. It's a dare. Impossible is nothing.
Affiliations: University of Chicago Booth School of Business
Joined: 03 Feb 2011
Posts: 709
Re: The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

08 Mar 2011, 09:43
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
Math Expert
Joined: 02 Sep 2009
Posts: 55277
Re: The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

16 May 2016, 22:04
GMATD11 wrote:
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

OPEN DISCUSSION OF THIS QUESTION IS HERE: the-product-of-all-the-prime-numbers-less-than-20-is-closest-135192.html

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

We should find the approximate value of 2*3*5*7*11*13*17*19 to some power of 10.

# of different approximations are possible.

Approach #1:

2*5=10;
3*7=~20 (actually more than 20);
11*19=~200 (actually more than 200);
13*17=~200 (actually more than 200);

$$2*3*5*7*11*13*17*19\approx{10*20*200*200=8*10^6}\approx{10^7}$$.

Approach #2:

2*5=10
3*17=~50 (actually more than 50);
7*13=~100 (actually less than 100);
11*19=~200 (actually more than 200)

$$2*3*5*7*11*13*17*19\approx{10*50*100*200}=10^7$$.

OPEN DISCUSSION OF THIS QUESTION IS HERE: the-product-of-all-the-prime-numbers-less-than-20-is-closest-135192.html
_________________
Retired Moderator
Joined: 04 Aug 2016
Posts: 483
Location: India
GPA: 4
WE: Engineering (Telecommunications)
Re: The product of all the prime numbers less than 20 is closest to which  [#permalink]

### Show Tags

05 Aug 2017, 00:28
Bunuel wrote:
GMATD11 wrote:
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

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/the-product-o ... 35192.html

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

We should find the approximate value of 2*3*5*7*11*13*17*19 to some power of 10.

# of different approximations are possible.

Approach #1:

2*5=10;
3*7=~20 (actually more than 20);
11*19=~200 (actually more than 200);
13*17=~200 (actually more than 200);

$$2*3*5*7*11*13*17*19\approx{10*20*200*200=8*10^6}\approx{10^7}$$.

Approach #2:

2*5=10
3*17=~50 (actually more than 50);
7*13=~100 (actually less than 100);
11*19=~200 (actually more than 200)

$$2*3*5*7*11*13*17*19\approx{10*50*100*200}=10^7$$.

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/the-product-o ... 35192.html

The OA is marked as 'D' for this question. Why do we have 2 threads with same question? Is it to keep number of posts readable?

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: The product of all the prime numbers less than 20 is closest to which   [#permalink] 05 Aug 2017, 00:28
Display posts from previous: Sort by

# The product of all the prime numbers less than 20 is closest to which

 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®.