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Cost is expressed by the formula tb^4

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Manager
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Joined: 13 Jul 2013
Posts: 73
GMAT 1: 570 Q46 V24
Cost is expressed by the formula tb^4 [#permalink]

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Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent of the original cost?

(A) 200
(B) 600
(C) 800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks
[Reveal] Spoiler: OA
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Re: Cost is expressed by the formula tb^4 [#permalink]

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New post 07 Sep 2013, 22:30
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theGame001 wrote:
Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent
of the original cost?

(A) 200
(B) 600
(C)800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks


The question asks for the relative percent b/w the new cost and the final cost, that is \(\frac{256}{16}*100\) and NOT the precent increase/decrease.

For example, initial cost=1 , final cost = 2, and the question asks what percent of the original cost is the final cost : \(\frac{2}{1}*100\) = 200%

Again, if the question asks, what is the percent increase between the final and the initial cost : \(\frac{2-1}{1}*100\) = 100%
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GMAT Date: 10-31-2013
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Re: Cost is expressed by the formula tb^4 [#permalink]

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New post 07 Sep 2013, 22:56
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KUDOS
theGame001 wrote:
Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent
of the original cost?

(A) 200
(B) 600
(C)800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks

Original Cost C1=t1*b1^4
New Cost C2=t2*b2^4....only b is doubled so t2=t1 and b2=2b1
C2=t2*(2b1)^4
=16(t1*b1^4)
=16C1
16 times C1=>1600% of C1
Ans D=1600
Manager
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Joined: 26 Feb 2013
Posts: 177
Re: Cost is expressed by the formula tb^4 [#permalink]

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New post 08 Sep 2013, 00:23
I thought we need to multiply b^4 and did 2(b^4) and answered wrong. :?
Manager
Manager
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Joined: 13 Jul 2013
Posts: 73
GMAT 1: 570 Q46 V24
Re: Cost is expressed by the formula tb^4 [#permalink]

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New post 08 Sep 2013, 02:15
mau5 wrote:
theGame001 wrote:
Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent
of the original cost?

(A) 200
(B) 600
(C)800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks


The question asks for the relative percent b/w the new cost and the final cost, that is \(\frac{256}{16}*100\) and NOT the precent increase/decrease.

For example, initial cost=1 , final cost = 2, and the question asks what percent of the original cost is the final cost : \(\frac{2}{1}*100\) = 200%

Again, if the question asks, what is the percent increase between the final and the initial cost : \(\frac{2-1}{1}*100\) = 100%


Ahhhh, okay. Thanks, I clearly misinterpreted the question.
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Re: Cost is expressed by the formula tb^4 [#permalink]

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New post 08 Sep 2013, 04:45
Expert's post
theGame001 wrote:
Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent of the original cost?

(A) 200
(B) 600
(C) 800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks


The point is that 16tb^4 is 1600% of tb^4 but 16tb^4 is 1500% greater than tb^4.
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Re: Cost is expressed by the formula tb^4   [#permalink] 08 Sep 2013, 04:45
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