Basic Exponent Question

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Basic Exponent Question

by Buix0065 » Thu Jan 27, 2011 6:54 pm
If 5^n > 10,000 and n is an integer, what is the smallest possible value of n?

Answer:
6

Please tell me your reasoning.

My Kaplan Premier book assumes that I can use a calculator on the GMAT... A little frustrating when I'm trying to read the explanation, for solving a problem.[/spoiler]
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by DarkKnight » Thu Jan 27, 2011 7:09 pm
5^n=10000
5^n=5^4 * 2^4

If n=5 then , 5^4*5 < 5^4 *16
for n=6, 5^4*5^2 > 5^4* 16.

Therefore minimum value of n=6.

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by GMATGuruNY » Thu Jan 27, 2011 7:51 pm
Buix0065 wrote:If 5^n > 10,000 and n is an integer, what is the smallest possible value of n?

Answer:
6

Please tell me your reasoning.

My Kaplan Premier book assumes that I can use a calculator on the GMAT... A little frustrating when I'm trying to read the explanation, for solving a problem.[/spoiler]
Another approach would be to ballpark:

5^3 = 125.
5^4 = 5*125 ≈ 600.
5^5 ≈ 5*600 = 3000.
5^6 ≈ 5*3000 = 15,000.

Thus, n=6 is the smallest value that will work. Since 5^5 is nowhere close to 10,000, we don't have to worry about being that exact.
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