Exponents

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Perminology: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Tue Sep 08, 2009 1:50 am; Thanked: 2 times; GMAT Score:540

by Perminology » Sat Oct 31, 2009 6:50 am

Did a practice test last night and this question really got to me

5^21 X 4^11=2X 10^n, n=?
11
21
22
23
32

Sat infront of the question for some time, and just chose 32 knowing it was wrong. Any one have a take on this?

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Source: Beat The GMAT — Problem Solving | Sat Oct 31, 2009 6:50 am

satish.nagdev: Master | Next Rank: 500 Posts; Posts: 130; Joined: Sat Dec 29, 2007 5:35 am; Location: SGP; Thanked: 5 times

by satish.nagdev » Sat Oct 31, 2009 7:30 am

should be 21
5^21 * 4^11 = 2 * 10^n
==> 5^21 * (2^2)^11 = 2 * (2^n * 5^n)
5^21 * 2^22 = 2^(n+1) * 5^n
so matching power of 5, n=21

whats o.a

Wounded by GMAT but not dead

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Perminology: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Tue Sep 08, 2009 1:50 am; Thanked: 2 times; GMAT Score:540

by Perminology » Sat Oct 31, 2009 12:11 pm

You are completely right. Thanks for the help!

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