Need a time saving solution

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sabal: Junior | Next Rank: 30 Posts; Posts: 11; Joined: Mon Aug 25, 2008 1:52 am

Need a time saving solution

by sabal » Mon Nov 24, 2008 1:41 am

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

The OA is C

This question is from OG 11 Diagnostic Test Q. No. 15

Does anyone have any idea on how to solve this besides doing the usual 2*3*5*....... A method wherein I can save some time...?

Thanks!!

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Source: Beat The GMAT — Problem Solving | Mon Nov 24, 2008 1:41 am

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pm; Thanked: 237 times; Followed by:25 members; GMAT Score:730

Re: Need a time saving solution

by logitech » Mon Nov 24, 2008 1:58 am

sabal 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

The OA is C

This question is from OG 11 Diagnostic Test Q. No. 15

Does anyone have any idea on how to solve this besides doing the usual 2*3*5*....... A method wherein I can save some time...?

Thanks!!

first of all you do not have many numbers to deal with

2, 3, 5. 7. 11. 13. 17. 19

2 and 5 = 10

3 and 7 = 20

11 and 19 = 200

17 and 13 =200

so the multiplication is around 8 x 10 ^ 6 which is close to 10 ^ 7

The fast way to approximate your results.

LGTCH
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