Good One from MGMAT Q Bank : GMAT Problem Solving (PS)
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# Good One from MGMAT Q Bank

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Retired Moderator
Status: The last round
Joined: 18 Jun 2009
Posts: 1310
Concentration: Strategy, General Management
GMAT 1: 680 Q48 V34
Followers: 79

Kudos [?]: 1026 [1] , given: 157

Good One from MGMAT Q Bank [#permalink]

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10 Jul 2010, 22:24
1
KUDOS
00:00

Difficulty:

(N/A)

Question Stats:

25% (01:11) correct 75% (00:44) wrong based on 10 sessions

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Which of the following best approximates the value of $$q$$ if 5^28 + 3^11=5^q ?

A 39
B 30
C 28
D 27
E 17
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Kaplan GMAT Instructor
Joined: 21 Jun 2010
Posts: 75
Location: Toronto
Followers: 29

Kudos [?]: 168 [3] , given: 2

Re: Good One from MGMAT Q Bank [#permalink]

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10 Jul 2010, 23:15
3
KUDOS
Note that the question invites approximation. ("which of the following approximates...?")

That combined with the fact that we are dealing with exponents that are so far apart is a huge signal that you should approach this one qualitatively, reasoning it out and approximating rather than using pure algebra.

If you understand how quickly orders of magnitude increase, then you can reason your way to the right answer very quickly. A number like 5^28 is going to be WAAAAAAY bigger than 3^11. Absolutely, hugely bigger than 3^11. So, 3^11 is negligible, like nothing, compared to 5^28. So, we can look at the equation like this:

5^28 + (nothing) = 5^q
5^28 = 5^q
28 = q

Choose C.
Retired Moderator
Status: The last round
Joined: 18 Jun 2009
Posts: 1310
Concentration: Strategy, General Management
GMAT 1: 680 Q48 V34
Followers: 79

Kudos [?]: 1026 [0], given: 157

Re: Good One from MGMAT Q Bank [#permalink]

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11 Jul 2010, 01:25
Thanks for the detailed input. OA is "C".
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Joined: 04 Feb 2010
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Kudos [?]: 41 [0], given: 8

Re: Good One from MGMAT Q Bank [#permalink]

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11 Jul 2010, 02:33
Wow, thanks for the explanation testluv. Been wondering what's the elegant solution to this.
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Joined: 25 Jan 2010
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Re: Good One from MGMAT Q Bank [#permalink]

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11 Jul 2010, 19:16
Thank you testluv!
Re: Good One from MGMAT Q Bank   [#permalink] 11 Jul 2010, 19:16
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