The cost of a square slab (GMAT Prep)

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Question from GMAT Prep

OA D

Would anyone please explain?

[tohellandback]

IMO D
From the question stem

Cost=K*T*L^2
K- a constant
T-thickness
L-Length
Given L=3, T=0.1, if we find K, we are done

1)K*0.2*4=K*0.1*4+160
we can find K, sufficient

2)K*0.1*9=K*0.1*4+200
again Sufficient

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Thanks I get it now. I didn't really get about the direct/invert propotional topic....thanks!