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# 10^180-10^30. Which of the following best approximates

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10^180-10^30. Which of the following best approximates [#permalink]  24 Sep 2009, 10:08
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10^180 - 10^30 =
Which of the following best approximates the value of the expression above?
(A) 10^180
(B) 10^179
(C) 10^170
(D) 10^160
(E) 10^150
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Re: GMAT, Number Properties, Exponents [#permalink]  24 Sep 2009, 11:03

10^180 - 10^30
= 10^30 (10^150 - 1)
~ 10 ^30 (10^150)
~ 10^180

This is more about the relative scale of the quantities than anything. It's easier to think about it with numbers you can picture, so assume you have an equation like 1 000 000 000 (10^7) - 100 (10^2). This equation can still be approximated as 10^7, since the latter term is so insignificantly small compared to the former term.
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Re: GMAT, Number Properties, Exponents [#permalink]  17 Jan 2012, 10:18
I found the answer options are:
a. 10^210
b. 10^180
c. 10^150
d. 10^90
e. 10^6

What is correct?
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Re: GMAT, Number Properties, Exponents [#permalink]  17 Jan 2012, 15:27
Baten80 wrote:
I found the answer options are:
a. 10^210
b. 10^180
c. 10^150
d. 10^90
e. 10^6

What is correct?

10^180 - 10^30 =
Which of the following best approximates the value of the expression above?

Yes, answer 10^180 is correct. Note that we need approximate value of the given expression and as 10^(180) is much larger number than 10^(30) then 10^(30) is pretty much negligible in this case: 10^(180)-10^(30)=~10^(180).

Check similar question for practice: tough-and-tricky-exponents-and-roots-questions-125956-40.html#p1029229

Hope it helps.
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Re: 10^180-10^30. Which of the following best approximates [#permalink]  17 Jan 2012, 20:17
+1 for B

Is this an actual question for somewhere? It feels ridiculously easy
Re: 10^180-10^30. Which of the following best approximates   [#permalink] 17 Jan 2012, 20:17
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