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# Multiplication the only way?

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Status: What to know what someone's dream looks like? Observe a large pile of GMAT books. (c)
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Multiplication the only way? [#permalink]  14 Dec 2010, 17:18
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Difficulty:

(N/A)

Question Stats:

67% (01:05) correct 33% (00:27) wrong based on 3 sessions
The product of all 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

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Joined: 03 Sep 2006
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Re: Multiplication the only way? [#permalink]  14 Dec 2010, 18:57
2*3*5*7*11*13*17*19

quick calculation:

3*10*77*13*17*19

231*10*13*17*19

231*10*221*19

231*221*190

Above steps did not take me much time, but after this I think one needs to be strong in multiplication!

If there is some short cut, it would be great to know....but I won't take the risk of some short cut which might result in wrong answer during the exam.

51051*190

=9699690

closer to 100,00000

which means 10^7
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
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Re: Multiplication the only way? [#permalink]  14 Dec 2010, 19:15
Expert's post

Quickly approximate
2, 3, 5, 7, 11, 13, 17, 19
Make groups
2*5 = 10
3*17 = 50 (approximately)
7*13 = 100 (approximately)
11*19 = 200 (approximately)
So you make 7 zeroes (the 2 and the 5 also make a 0). When you multiply all these, the answer will be close to 10^7
_________________

Karishma
Veritas Prep | GMAT Instructor
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Intern Joined: 15 Nov 2010 Posts: 20 Followers: 0 Kudos [?]: 0 [0], given: 0 Re: Multiplication the only way? [#permalink] 15 Dec 2010, 05:50 "My response from another thread: Quickly approximate 2, 3, 5, 7, 11, 13, 17, 19 Make groups 2*5 = 10 3*17 = 50 (approximately) 7*13 = 100 (approximately) 11*19 = 200 (approximately) So you make 7 zeroes (the 2 and the 5 also make a 0). When you multiply all these, the answer will be close to 10^7 " Dear Karishma, a quick question on this: when i actually multiply the numbers, i get 10^6..there are only 6 zeros from what I see. What am I overlooking ? Thank you in advance! Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 6238 Location: Pune, India Followers: 1683 Kudos [?]: 9657 [0], given: 198 Re: Multiplication the only way? [#permalink] 15 Dec 2010, 06:05 Expert's post 0987654312 wrote: "My response from another thread: Quickly approximate 2, 3, 5, 7, 11, 13, 17, 19 Make groups 2*5 = 10 3*17 = 50 (approximately) 7*13 = 100 (approximately) 11*19 = 200 (approximately) So you make 7 zeroes (the 2 and the 5 also make a 0). When you multiply all these, the answer will be close to 10^7 " Dear Karishma, a quick question on this: when i actually multiply the numbers, i get 10^6..there are only 6 zeros from what I see. What am I overlooking ? Thank you in advance! The 5 of 50 and the 2 of 200 above multiply to make another 0. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: Multiplication the only way?   [#permalink] 15 Dec 2010, 06:05
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