Another large exponent problem

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cooperbaker: Junior | Next Rank: 30 Posts; Posts: 10; Joined: Mon Dec 15, 2008 6:27 pm; Location: Atlanta, GA

by cooperbaker » Sun Jan 25, 2009 2:06 pm

What is (10^8 - 10^2)/(10^7 - 10^3)?

a) 1
b) 10
c) 10^2
d) 10^3
e) 10^4

OA: B

Try to use some verbage to explain if possible, I don't always understand everyone's math simplifications and shortcuts.

thanks.

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Source: Beat The GMAT — Problem Solving | Sun Jan 25, 2009 2:06 pm

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Sun Jan 25, 2009 2:47 pm

This question asks you to estimate the answer, not find the exact answer. If you look at the numerator, 10^8 is equal to 100 million, while 10^2 is just one hundred. So the numerator is almost exactly equal to 100 million. In the denominator, 10^7 is 10 million, while 10^3 is one thousand, so the denominator is almost exactly equal to 10 million. That is, we'll get an excellent estimate of the fraction's value just by considering the terms with the largest exponents: 10^8/10^7 = 10.

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cooperbaker: Junior | Next Rank: 30 Posts; Posts: 10; Joined: Mon Dec 15, 2008 6:27 pm; Location: Atlanta, GA

by cooperbaker » Sun Jan 25, 2009 2:58 pm

Makes sense -- thanks!

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gaggleofgirls: Master | Next Rank: 500 Posts; Posts: 138; Joined: Thu Jan 15, 2009 7:52 am; Location: Steamboat Springs, CO; Thanked: 15 times

by gaggleofgirls » Sun Jan 25, 2009 5:46 pm

I don't understand how "What is x/y?" is asking us to estimate?

If the question was "x/y is nearest to?" or "x/y equals approximately?" then I would have gotten the estimate part.

-Carrie

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cooperbaker: Junior | Next Rank: 30 Posts; Posts: 10; Joined: Mon Dec 15, 2008 6:27 pm; Location: Atlanta, GA

by cooperbaker » Sun Jan 25, 2009 6:07 pm

gaggleofgirls wrote:I don't understand how "What is x/y?" is asking us to estimate?

If the question was "x/y is nearest to?" or "x/y equals approximately?" then I would have gotten the estimate part.

-Carrie

Carrie -- after looking back at the question it did ask to approximate, in my haste to find an answer I just typed out the problem without the proper wording. Sorry for confusion.

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gaggleofgirls: Master | Next Rank: 500 Posts; Posts: 138; Joined: Thu Jan 15, 2009 7:52 am; Location: Steamboat Springs, CO; Thanked: 15 times

by gaggleofgirls » Sun Jan 25, 2009 6:13 pm

Ah, thank you. With the estimate in the question, then 10 number created by the large exponent (10^8 and 10^7) so dwarf the numbers created byt the small exponents, that they can be discarded and 10^8 / 10^7 = 18^8-7 = 10

-Carrie

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