stellategang, I will go for D.
Say,
c = cost of the wooden slab
l = length
d = depth
According to the Qs,
C <proportional> l and c <proportional> d
so c <proportional> ld
c = kld [K = constant]

Now by stmt - 1, .4k = 160 + .2k
And by stmt - 2, .3k = 200 + .2k
From each of the equs, we can find out the value of k., hence the cost as well.
Got me, stellategang?

Hey Amitiva that is really a very nice approach!!

congrats!

when it says "proportional to the square of its length" doesn't this mean:

c = kl^2d (what i mean is shouldn't "l" be squared?)

therefore stmt 1: 0.8k = 160 + 0.2k
and stmt 2: 0.6k = 200 + 0.2k

or am i missing something?

Here's how I would do this one.

To answer the question, you need to know the cost per unit of volume.

The first statement,
- change in volume ( 2*2*.2 )-(2*2*.1) = ( .8 )-( .4 ) = .4
- change in cost ($160) / .4 = $400
Each .1 cubic meters is $100

The second statement,
- change in volume ( 3*3*.1 )-( 2*2*.1 ) = ( .9 )-( .4 ) = .5
- change in cost ($200) / .5 = $400
Each .1 cubic meters is $100

Both give you the same cost per unit.
Answer is D