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Need a time saving solution

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