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Director
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The cost of a square slab of concrete is proportional to the [#permalink]
 21 Jul 2009, 15:28
Question Stats:
66% (00:00) correct
33% (01:38) wrong based on 1 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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Current Student
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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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Manager
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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 PS.: It was worth? Consider a kudo... 
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Intern
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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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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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Intern
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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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I wrote the formula as Cost = aL^2 + bT, so I got D.
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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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