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The cost of a square slab is proportional to its thickness a [#permalink]

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19 Jul 2008, 21:29

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The cost of a square slab is proportional to its thickness and also proportional to the square of its length. What is the cost of a square slab that is 3 meters long and 0.1m thick.

(1) The cost of a square slab that is 2 meters long and 0.2 m thick is $160 more than the cost of a slab that is 2m long and 0.1 m thick

(2) The cost of a square slab that is 3 meters long and 0.1 m thick is 200 more than the cost of a square slab that is 2m long and 0.1 m thick

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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19 Jul 2008, 21:56

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The cost of a square slab is proportional to its thickness and also proportional to the square of its length. What is the cost of a square slab that is 3 meters long and 0.1m thick.

(1) The cost of a square slab that is 2m long and 0.2 m thick is $160 more than the cost of a slab that is 2m long and 0.1 m thick area1 = 2x2x0.2 = 0.4m^2; area2 = 2x2x0.1 = 0.2m^2 A1 - A2 = 0.2m^2 = $160; so you can calculate the area of 0.1m^2 and you know that the are of the salb in question is 3x3x.1 = 0.9m^2

(2) The cost of a square slab that is 3 m long and 0.1 m thick is 200 more than the cost of a square slab that is 2m long and 0.1 m thick Follow same logic as S1.

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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07 Sep 2009, 12:50

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One clue for my view: the cost can be proportional to both thickness and length but with different proportionality constants. I mean, to me: Cost=a*thickness+b*length^2 not Cost=a*(thickness+length^2)

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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07 Sep 2009, 14:08

LenaA wrote:

I think the cost function is the following:

\(C=k\times t\times l^2\) t-thickness l-length

then each statement alone is sufficient

stmt1 \(4\times 0.2\times k=4\times 0.1\times k +160\) you can solve for k sufficient

stmt2 is basically similar to stmt 1... \(9\times 0.1\times k=4\times 0.1\times k +200\) you cansolve for k sufficient

In fact, that is the formula in order to be D the correct answer (as it is). But my point is, that in a very strict point of view, the proportionality constant (what you mean k), can be different for t and l, that is: Cost=k1*t+k2*l^2. So you need both statements to solve for k1 and k2, and correct answer is C.

To sum up, correct answer is C. OA is D.
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Re: The cost of a square slab is proportional to its thickness a [#permalink]

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07 Sep 2009, 15:06

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you are wrong. i would suggest to research about the jointly proportional functions. if z is proptional to x (when y is constant) and z is propotional to y (when x is constant), then z is propotional to the product xy and is of the form z=Kxy

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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07 Sep 2009, 15:20

LenaA wrote:

you are wrong. i would suggest to research about the jointly proportional functions. if z is proptional to x (when y is constant) and z is propotional to y (when x is constant), then z is propotional to the product xy and is of the form z=Kxy

Correct. Thank u. Dont know in what i was thinking about!

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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07 Sep 2009, 15:32

We have only one value missing (here it's K) so it's sure that each statement is sufficient.

noboru wrote:

LenaA wrote:

you are wrong. i would suggest to research about the jointly proportional functions. if z is proptional to x (when y is constant) and z is propotional to y (when x is constant), then z is propotional to the product xy and is of the form z=Kxy

Correct. Thank u. Dont know in what i was thinking about!

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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19 Jan 2013, 03:01

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kiyo0610 wrote:

The cost of a square slab is proportional to its thickness and also proportional to the square of its length. What is the cost of a square slab that is 3 meters long and 0.1 meter thick?

(1)The cost of a square slab that is 2 meters long and 0.2 meter thick is $160 more than the cost of a square slab that is 2 meters long and 0.1 meter thick. (2)The cost of a square slab that is 3 meters long and 0.1 meter thick is $200 more than the cost of a square slab that is 2 meters long and 0.1 meter thick.

Cost C 1) C proportional to Thickness t 2) C proportional to Length square l^2

C = K t l^2

We need to know constant K to find the answer.

Option 1: C1 and C2 difference is given for some thickness and length. We can find the constant Option 2: Same as option1
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Re: The cost of a square slab is proportional to its thickness a [#permalink]

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09 Sep 2013, 07:04

I formed equation for cost as :

C prop to l^2 C prop to t

C = kl^2 + rt l= for length t= for thickness. k and r constant of respective proportionality.

But in above mentioned solution it is taken as product.

I am not 100% satisfied with the derived proportionality as the product of length and thickness.

May be I am not able to identify the keyword in the question which governs product of two variables. Or lacking some basic concept, kindly help me to interpret the language of question into a equation. Please also share any theoretical stuff, which I should refer to understand concept of proportionality.

Thanks
_________________

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Re: The cost of a square slab is proportional to its thickness a [#permalink]

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11 Sep 2013, 05:12

When I first attempted to solve this problem I was a little thrown off by the question just saying proportional, and not directly proportional or indirectly proportional. I now realize that solving this problem is independent of the direct vs. indirect, you may get different values for the cost, but regardless you'll be able to get a value => sufficient.

My question is, can you assume that it's directly proportional from the question stem? Looking at a few of the answers above, it seems that some people have. If this was a P.S. problem instead of a D.S., the answer would depend on this assumption.

Re: The cost of a square slab is proportional to its thickness a [#permalink]

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25 Nov 2013, 03:01

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Interpreting the infamous stone slab word problem [#permalink]

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17 Jan 2015, 18:50

The cost of a square slab is proportional to its thickness and also proportional to the square of its length. What is the cost of a square slab that is 3 meters long and 0.1m thick.

(1) The cost of a square slab that is 2 meters long and 0.2 m thick is $160 more than the cost of a slab that is 2m long and 0.1 m thick

(2) The cost of a square slab that is 3 meters long and 0.1 m thick is 200 more than the cost of a square slab that is 2m long and 0.1 m thick

I read over this on multiple forums and have come to understand why the correct answer is correct.

That is this question can be written as C = kAT where C is cost, A is area, and T is thickness. The wording of the problem essentially states that C is jointly proportional to A and T.

I (and I think a few others) chose to interpret the question as C = kA + mT, where there are now two proportionality constants defining the relationship. At first glance this seems like what the question is leading into, but alas is not the OA.

So based on the original wording of the question we can surmise the relationship is C=kAT. But what wording do you use then to describe the second relationship C = kA + mT? This way I know how to distinguish between these two types of relationships described.

Re: Interpreting the infamous stone slab word problem [#permalink]

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18 Jan 2015, 01:17

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Kevin, In the case you mentioned, the wording should be something like "The cost of stone slab is dependent on its area and height" this can be interpreted as "C= kA + mT

But when it is mentioned that cost is "proportional" to any particular factor, then that implies a multiplicative relation only.

Further, on a lighter note: in the relation C= kA+mT; there can still be a cost even when one of A or T is zero!! So, I feel that even this fact indicates toward a relation like C= kAT