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Math Expert V
Joined: 02 Sep 2009
Posts: 59561
If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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Question Stats: 73% (02:12) correct 27% (02:47) wrong based on 1187 sessions

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If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)
Math Expert V
Joined: 02 Aug 2009
Posts: 8282
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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3
1
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4a}$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Bunuel, pl relook ..$$b = 1 + \frac{1}{4a}$$ should be $$b = 1 + \frac{1}{4}*a$$
otherwise you will not get the denominator as a multiple of 4

$$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}=1+\frac{1}{2^2}+\frac{1}{2^4}+\frac{1}{2^6}$$...
it is a geometric progression
sum = $$\frac{a(1-r^n)}{1-r}=1*(1-\frac{1}{4}^4)/(1-\frac{1}{4})=\frac{255}{256}/\frac{3}{4}=\frac{255*4}{256*3}=\frac{85}{64}$$...

$$b = 1 + \frac{a}{4}=1+\frac{85}{64*4}=\frac{341}{256}$$....

$$a-b=\frac{85}{64}-\frac{341}{256}=\frac{340-341}{256}=\frac{-1}{256}$$

B
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Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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7
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

a = 1 + 1/4 + 1/16 + 1/64
a = ( 64 + 16 + 4 + 1 ) / 64
a = 85 / 64

Now,
b = 1 + 1/4 * 85/64
b = 1 + 85/256
b = 341/256

Therefore,
a - b = 85/64 - 341/256
a - b = ( 85*4 - 341 ) / 256
a - b = (340 - 341)/256
a - b = - 1 / 256

Hence, B.
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Target Test Prep Representative V
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Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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4
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

a - b

a - (1 + ¼a)

¾a - 1

¾(1 + ¼ + 1/16 + 1/64) - 1

¾(64/64 + 16/64 + 4/64 + 1/64) - 1

¾(85/64) - 1

255/256 - 256/256

-1/256

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Joined: 11 Oct 2018
Posts: 21
Location: Germany
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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3
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Hi,

this is my first post. Hope I did it correctly.

$$a=1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}$$
$$b=1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}+\frac{1}{256}$$

$$a-b=1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}-1-\frac{1}{4}-\frac{1}{16}-\frac{1}{64}-\frac{1}{256}$$
or
$$a-b=1-1+\frac{1}{4}-\frac{1}{4}+\frac{1}{16}-\frac{1}{16}+\frac{1}{64}-\frac{1}{64}-\frac{1}{256}$$

Just simplify:

$$=-\frac{1}{256}$$
Intern  Joined: 11 Jul 2018
Posts: 19
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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2
Value of a = 1 + 1/4 = 1.25 (ignoring other bit which will make it slightly bigger than 1.25)
Value of b = 1 + 1/(4*1.25) = 1 + 1/5 = 1.20

Roughly a - b = 1.25 - 1.20 = .05 => Actual value of a will be slightly more than 1.25 and therefore, value of b will be slightly less than what is present. This will mean this difference will slightly bigger but not drastically big.

I will go with smaller option of D.

This is not a proper way to solve this query as it can be solve using equations or GP series etc in a proper manner but that will be too time consuming in exam.
Intern  B
Joined: 07 Feb 2017
Posts: 25
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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1
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Easier way to do rather than using Geo series formula is using substitution method.

$$b=1+\frac{1}{4} a$$
$$b=a+\frac{1}{256}$$
so, $$b-a = -\frac{1}{256}$$
Senior RC Moderator V
Joined: 02 Nov 2016
Posts: 4554
GPA: 3.39
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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1
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Bunuel

I think this question is already appeared in OG Quantitative review 2018. Kindly verify
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Manager  B
Joined: 19 Jan 2018
Posts: 84
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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1
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Simple solution to this problem if you apply logic:
We know that
$$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and
$$b = 1 + \frac{1}{4}a$$
We're solving for a - b
Plug a in for b so you get the following:

($$1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$) - ($$1 + \frac{1}{4}[1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$]

Distrubute $$\frac{1}{4}$$

($$1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$) - ($$1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64} + \frac{1}{256}$$]

The values in Red cross out, leaving you with $$\frac{-1}{256}$$
($$1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$) - ($$1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ + $$\frac{1}{256}$$]

Math Expert V
Joined: 02 Aug 2009
Posts: 8282
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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1
StudiosTom wrote:
chetan2u wrote:
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4a}$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Bunuel, pl relook ..$$b = 1 + \frac{1}{4a}$$ should be $$b = 1 + \frac{1}{4}*a$$
otherwise you will not get the denominator as a multiple of 4

$$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}=1+\frac{1}{2^2}+\frac{1}{2^4}+\frac{1}{2^6}$$...
it is a geometric progression
sum = $$\frac{a(1-r^n)}{1-r}=1*(1-\frac{1}{4}^4)/(1-\frac{1}{4})=\frac{255}{256}/\frac{3}{4}=\frac{255*4}{256*3}=\frac{85}{64}$$...

$$b = 1 + \frac{a}{4}=1+\frac{85}{64*4}=\frac{341}{256}$$....

$$a-b=\frac{85}{64}-\frac{341}{256}=\frac{340-341}{256}=\frac{-1}{256}$$

B

Hi chetan2u, Can you please elaborate the geometric progression formula aind its application.

HI,

A GP is a series where each successive number is SOME times the preceding term and this SOME could be any number. This is also referred as the RATIO or simply r. so if a, b, c,d ois the series in GP, b/a=r...
If 2, 4, 6, 8..r=4/2=2
If 4, 2, 1, 1/2....r=2/4=1/2.

There can be various applications which meets this requirement. Say a certain species doubles itself every year, or a certain amount of money increases by 10% every 4 months and so on.
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Math Expert V
Joined: 02 Sep 2009
Posts: 59561
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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chetan2u wrote:
Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4a}$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Bunuel, pl relook ..$$b = 1 + \frac{1}{4a}$$ should be $$b = 1 + \frac{1}{4}*a$$
otherwise you will not get the denominator as a multiple of 4

$$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}=1+\frac{1}{2^2}+\frac{1}{2^4}+\frac{1}{2^6}$$...
it is a geometric progression
sum = $$\frac{a(1-r^n)}{1-r}=1*(1-\frac{1}{4}^4)/(1-\frac{1}{4})=\frac{255}{256}/\frac{3}{4}=\frac{255*4}{256*3}=\frac{85}{64}$$...

$$b = 1 + \frac{a}{4}=1+\frac{85}{64*4}=\frac{341}{256}$$....

$$a-b=\frac{85}{64}-\frac{341}{256}=\frac{340-341}{256}=\frac{-1}{256}$$

B

_______________
Edited. Thank you.
Intern  B
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Joined: 22 Jul 2018
Posts: 3
Concentration: Technology, Entrepreneurship
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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can this be done in 2 minutes?
Manager  S
Joined: 16 Mar 2017
Posts: 61
Location: France
Concentration: Marketing, Strategy
GMAT 1: 640 Q38 V40 GMAT 2: 710 Q47 V41 WE: Marketing (Retail)
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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whatfielddoido wrote:
can this be done in 2 minutes?

Apparently yes "73% (01:44) correct"
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Concentration: Finance, Statistics
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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2
How I solved it in 30s.

a= 1+0.25+(small numbers)=almost 1.3
b= 1+0,25*1.3=1+ slightly more than 0.3.
a - b= (less than) 1.3 - (more than) 1.3 = negative, but close to 0.

Answer options allow for B as best answer choice. B is correct.
Manager  B
Joined: 19 Aug 2016
Posts: 73
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293)

Hi Bunuel,

I took the LCM of b=1+1/4a

Then b became 5/4a

I got the answer as -85/256

Why can't we take the LCM and do it this way?

And if we can then either way, we must get the same answer..

Director  D
Status: Learning stage
Joined: 01 Oct 2017
Posts: 991
WE: Supply Chain Management (Energy and Utilities)
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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Hi Bunuel,

I took the LCM of b=1+1/4a

Then b became 5/4a

I got the answer as -85/256

Why can't we take the LCM and do it this way?

And if we can then either way, we must get the same answer..

Hi zanaik89,
Refer the highlighted portion, It should be :
$$b=1+\frac{1}{4}a$$-------------(2)
=$$\frac{4+a}{4}$$ (Though we don't require this step)
a=1+$$\frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$=$$\frac{64+16+4+1}{64}$$=$$\frac{85}{64}$$------------(1)

Hence $$a-b=\frac{85}{64}-(1+\frac{1}{4}*\frac{85}{64})=\frac{85}{64}-1-\frac{85}{4*64}=\frac{(4*85)-(4*64)-85}{4*64}$$=-$$\frac{1}{256}$$

Another Method:-
$$a=1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}$$
So, $$\frac{1}{4}a=\frac{1}{4}+\frac{1}{16}+\frac{1}{64}+\frac{1}{256}$$ (Dividing both sides by 4)
So, $$b=1+\frac{1}{4}a=1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}+\frac{1}{256}$$

Hence, $$a-b=(1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64})-(1+\frac{1}{4}+\frac{1}{16}+\frac{1}{64}+\frac{1}{256})$$
Or, $$a−b=−\frac{1}{256}$$
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Intern  B
Joined: 18 Jun 2017
Posts: 8
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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Hello to everyone!
Is there any way to solve this task in 1 minute, please? Any tricks or magic?

The task is not complicated, but the problem takes more than 1.5 to be solved out.

Thanks.
Intern  B
Joined: 01 May 2017
Posts: 34
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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get value of A and B then substitute, you will easily get the answer.
Intern  B
Joined: 24 Apr 2018
Posts: 1
If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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a=1+1/4+1/4^2 +1/4^3

B=1+1/4a , substitue vaule a in to b

b= 1+1/4(1+1/4+1/4^2 +1/4^3 )
= 1+1/4+1/4^2 +1/4^3 +1/4^4

a-b= 1+1/4+1/4^2 +1/4^3 - (1+1/4+1/4^2 +1/4^3 +1/4^4 )

=-1/4^4
=-1/256
Intern  B
Joined: 26 May 2019
Posts: 16
WE: Accounting (Accounting)
Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value  [#permalink]

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Tried both geometric progression method and substitution, and substitution was faster for me.

Geometric progression just had too much calculation with all the fractions; substitution reduced the calculations.

Given:
$$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64} = \frac{85}{64}$$
$$b = 1 + (\frac{1}{4})*a$$

Substitution Method:
a - b
$$= \frac{85}{64} - (1 + (\frac{1}{4})*(\frac{85}{64}))$$
$$= \frac{85}{64} - \frac{64}{64} - (\frac{1}{4})*(\frac{85}{64})$$
$$= \frac{21}{64} - (\frac{1}{4})*(\frac{85}{64})$$
$$= \frac{84}{256} - \frac{85}{256}$$
$$= \frac{-1}{256}$$

Answer is B $$\frac{-1}{256}$$

Bunuel wrote:
If $$a = 1 + \frac{1}{4} + \frac{1}{16} + \frac{1}{64}$$ and $$b = 1 + \frac{1}{4}a$$, then what is the value of a – b ?

A. -85/256

B. -1/256

C. -1/4

D. 125/256

E. 169/256

NEW question from GMAT® Quantitative Review 2019

(PS14293) Re: If a = 1 + 1/4 + 1/16 + 1/64 and b = 1 + 1/4a, then what is the value   [#permalink] 04 Aug 2019, 19:19

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