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Missed Easy Algebra Question

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by drewd313 » Mon Jan 05, 2009 7:39 am
So, I did the official GMAT practice test and missed the very first question! The worst part is that I can't figure out how to correctly solve the problem.

5^21 X 4^10 = 2 X 10^n, what is the value of n?

I solved by taking the above equation to equal 20^31 = 20^n, and thus
n = 31. Which is not the answer.

Can someone explain to me how to correctly solve the problem? Thank you.
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by DavoodBeater » Mon Jan 05, 2009 7:59 am
are you sure that you wrote the question right? If it s write, it is really hard!
but if the right side is 1/2 instead of 2 the answer is 21 and it is really easy!
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Re: Missed Easy Algebra Question

by Brent@GMATPrepNow » Mon Jan 05, 2009 8:43 am
drewd313 wrote:So, I did the official GMAT practice test and missed the very first question! The worst part is that I can't figure out how to correctly solve the problem.

5^21 X 4^10 = 2 X 10^n, what is the value of n?

I solved by taking the above equation to equal 20^31 = 20^n, and thus
n = 31. Which is not the answer.

Can someone explain to me how to correctly solve the problem? Thank you.
The question is correct as posted. However, you combined the powers even though the bases are not equal.
The law (x^n)(x^m) = x^(m+n) only applies when you have the same denominator.

Here are a few solutions to the question: https://www.beatthegmat.com/exponents-gm ... t3395.html
https://www.beatthegmat.com/exponent-gmatprep-t2207.html

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by DavoodBeater » Mon Jan 05, 2009 9:17 am
still the question is wrong, i dont mean wrong, but not easy to handle and not an integer at least.

the two link are to this question 5^21X4^11 = 2X10^n which has 4^11 instead of 4^10.
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by Brent@GMATPrepNow » Mon Jan 05, 2009 11:05 am
DavoodBeater wrote:still the question is wrong, i dont mean wrong, but not easy to handle and not an integer at least.

the two link are to this question 5^21X4^11 = 2X10^n which has 4^11 instead of 4^10.
Sorry, I didn't notice that part. You're right.
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by drewd313 » Mon Jan 05, 2009 12:16 pm
DavoodBeater wrote:still the question is wrong, i dont mean wrong, but not easy to handle and not an integer at least.

the two link are to this question 5^21X4^11 = 2X10^n which has 4^11 instead of 4^10.
You are right, I posted the wrong question. It should be the same as the one in the two links. I am on a different computer then I took the test so I didn't remember it exactly.

Thanks for the responses!
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