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If x= 2^b - (8^8 + 8^6), for which of the following b values

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If x= 2^b - (8^8 + 8^6), for which of the following b values [#permalink] New post 20 Sep 2013, 10:12
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B
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65% (02:30) correct 35% (01:37) wrong based on 83 sessions
If x= 2^b - (8^8 + 8^6), for which of the following b values is x closest to 0?

(A) 20
(B) 24
(C) 25
(D) 30
(E) 42
[Reveal] Spoiler: OA

Last edited by Bunuel on 20 Sep 2013, 10:18, edited 1 time in total.
RENAMED THE TOPIC.
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Re: If x= 2^b - (8^8 + 8^6), for which of the following b values [#permalink] New post 20 Sep 2013, 10:23
Expert's post
abhisheksharma85 wrote:
If x= 2^b - (8^8 + 8^6), for which of the following b values is x closest to 0?

(A) 20
(B) 24
(C) 25
(D) 30
(E) 42


8^8 + 8^6=8^2*8^6 + 8^6=64*8^6+8^6=65*8^6=65*2^{18}\approx{2^{6}*2^{18}}=2^{24}.

2^b - (8^8 + 8^6)=0 --> 2^b - 2^{24}=0 --> b=24

Answer: B.

OR: 8^8 + 8^6 is very close to 8^8=2^24, thus b=24.
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Re: If x= 2^b - (8^8 + 8^6), for which of the following b values [#permalink] New post 20 Sep 2013, 10:27
Expert's post
abhisheksharma85 wrote:
If x= 2^b - (8^8 + 8^6), for which of the following b values is x closest to 0?

(A) 20
(B) 24
(C) 25
(D) 30
(E) 42

Dear abhisheksharma85,
I'm happy to help with this. :-)

Recall the exponent rule:
(a^m)^n = a^(m*n)

Therefore,
8^8 + 8^6 = (2^3)^8 + (2^3)^6 = 2^24 + 2^18

This is the thing we want to cancel with 2^b. Well, keep in mind that 2^24 is 2^6 = 64 times larger than 2^18, so for the purposes of cancelling, we will ignore the tinier number. If we want to cancel the 2^24, we need b = 24, answer = (B).

Does all this make sense?
Mike :-)
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Re: If x= 2^b - (8^8 + 8^6), for which of the following b values [#permalink] New post 20 Sep 2013, 10:31
Expert's post
Bunuel wrote:
abhisheksharma85 wrote:
If x= 2^b - (8^8 + 8^6), for which of the following b values is x closest to 0?

(A) 20
(B) 24
(C) 25
(D) 30
(E) 42


8^8 + 8^6=8^2*8^6 + 8^6=64*8^6+8^6=65*8^6=65*2^{18}\approx{2^{6}*2^{18}}=2^{24}.

2^b - (8^8 + 8^6)=0 --> 2^b - 2^{24}=0 --> b=24

Answer: B.

OR: 8^8 + 8^6 is very close to 8^8=2^24, thus b=24.


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Hope it helps.
_________________

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PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If x= 2^b - (8^8 + 8^6), for which of the following b values [#permalink] New post 26 Feb 2014, 20:08
8^8 = 2^24
8^6 = 2^18

2^24 + 2^18 is nearly to 2^24, so b=24 = Answer = B
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Re: If x= 2^b - (8^8 + 8^6), for which of the following b values   [#permalink] 26 Feb 2014, 20:08
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