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The cost of a square slab of concrete is proportional to the

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The cost of a square slab of concrete is proportional to the [#permalink] New post 21 Jul 2009, 14:28
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57% (00:00) correct 43% (01:34) wrong based on 11 sessions
The cost of a square slab of concrete is proportional to the thickness and also proportional to the square of the length. What is the cost of square slab that is 3 meters long and 0.1 meters thick?
a. 2m long by 0.2m thick is $160 more than 2m long by 0.1m thick
b. 3m long by 0.1m thick is $200 more than 2m long by 0.1m thick
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Re: GMATPrep1 [#permalink] New post 21 Jul 2009, 14:59
Stmt1 - 2m long by 0.2m thick is $160 more than 2m long by 0.1m thick

Now since the cost is directly porportional to the thickness. Lets imagine the "2m long by 0.2m thick" slab as two "2m long by 0.1m thick" slabs

we have "2m long by 0.2m thick" = 160 + "2m long by 0.1m thick"

"2m long by 0.2m thick" can be written as "2m long by 0.1m thick" + "2m long by 0.1m thick"

therefore "2m long by 0.1m thick" + "2m long by 0.1m thick" = 160 + "2m long by 0.1m thick"

therefore "2m long by 0.1m thick" = $160 .... 1

We cant deduce from stmt 1 ... insuff


Stmt 2) 3m long by 0.1m thick is $200 more than 2m long by 0.1m thick

3m long by 0.1m thick = $200 + 2m long by 0.1m thick .... 2

stmt2 insuffecient

combing 1 and 2

we have 3m long by 0.1m thick = $200 + $160(from 1)

= $360 ...


therefore answer is C

Please confirm
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Re: GMATPrep1 [#permalink] New post 21 Jul 2009, 15:02
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Thickness = t
Length = l
c = 9*0.1*k = ?

"and also proportional" means that we can put both in the same formula:

Cost = k*t*l^2, where k is a constant

1)
k*2^2*0.1 costs x
k*2^2*0.2 costs x + 160
Using rule of three (cross-multiplication): (k*2^2*0.1)/(k*2^2*0.2)= x/( x + 160 ), you can find x, and then do the same with 9*0.1*k
SUFFICIENT

2)
Using the same reasoning of 1)
SUFFICIENT

Answer D

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Re: GMATPrep1 [#permalink] New post 20 Dec 2009, 12:46
please check if this method would work as well since I didn't relaly get coelholds answer (and i get a negative solution for (b) with his method...)

the cost is proportional to the area and the thickness

The cost: C
We're looking for (3^2*0,1) C = ?

(a) (2^2*0,2)*C = (2^2*0,1)*C + 160
0,8*C = 0,4*C + 160
0,4C = 160
C = 400

(b) (3^2*0,1)*C = (2^2*0,1)C + 200
0,9C = 0,4C + 200
0,5C = 200
C = 400

Hence, D.
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Re: GMATPrep1 [#permalink] New post 20 Dec 2009, 21:32
The key with such questions is to set up the formula based on the proportionality.

Cost = K*(L^2)*T
Where K is a constant
L is the length
T is the thickness
Solving we get D

Note is something is inversely proportional, divide instead of multiply. For example if the cost is inversely proportional to the square root of slab impurity then the above formula will become
Cost = K*(L^2)*T/sqrt(I)
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Re: GMATPrep1 [#permalink] New post 20 Dec 2009, 21:53
really a tricky one! i initially thought it to be C but then checked the formula and found it to be D.
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Re: GMATPrep1 [#permalink] New post 15 Oct 2011, 13:17
I wrote the formula as Cost = aL^2 + bT, so I got D.
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Re: GMATPrep1 [#permalink] New post 16 Oct 2011, 09:41
seofah wrote:
The cost of a square slab of concrete is proportional to the thickness and also proportional to the square of the length. What is the cost of square slab that is 3 meters long and 0.1 meters thick?
a. 2m long by 0.2m thick is $160 more than 2m long by 0.1m thick
b. 3m long by 0.1m thick is $200 more than 2m long by 0.1m thick



Got D but worked as following:

a. 2^2L + .2W = 2^2L + .1W + 160 => SUFFICIENT (Solve for W and L)
b. 3^2L + .1W = 2^2L + .1W + 200 => SUFFICIENT (Solve for W and L)

Wonder if this a right approach to attack such question.
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Re: GMATPrep1   [#permalink] 16 Oct 2011, 09:41
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