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# multiply numbers

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Senior Manager
Joined: 10 Nov 2010
Posts: 266
Location: India
Concentration: Strategy, Operations
GMAT 1: 520 Q42 V19
GMAT 2: 540 Q44 V21
WE: Information Technology (Computer Software)
Followers: 5

Kudos [?]: 169 [0], given: 22

multiply numbers [#permalink]  08 Mar 2011, 07:34
00:00

Difficulty:

(N/A)

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
Joined: 02 Apr 2010
Posts: 103
Followers: 5

Kudos [?]: 108 [0], given: 18

Re: multiply numbers [#permalink]  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
Joined: 14 Feb 2011
Posts: 196
Followers: 4

Kudos [?]: 95 [0], given: 3

Re: multiply numbers [#permalink]  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
Status: Matriculating
Affiliations: Chicago Booth Class of 2015
Joined: 03 Feb 2011
Posts: 920
Followers: 13

Kudos [?]: 272 [0], given: 123

Re: multiply numbers [#permalink]  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
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