Take a peek at Section 2, number 13 of the June 2007 LSAT, but stop before you get to the answer choices. Pretend you're a litigator and the argument is being made by your opposition, an evil soft-drink executive. Who is suing whom here, and over what, doesn't really matter. What does matter is that the evil soft drink executive is your opponent, and you need to try to discredit his argument. Does the argument on the page make sense, or is it bullshit? In other words, what is he trying to prove here? Do his facts justify the conclusion he is trying to reach? Or is there something missing? If you were cross-examining this guy, what question(s) might you ask?
The conclusion seems to be "M contains twice as many cans as L." Do we agree with this conclusion, or do we disagree? Of course, we always disagree. Be skeptical! The evidence provided is:
1) M contains 50% recycled aluminum.
2) All of the recycled aluminum in M came from L.
3) Aluminum cans are made almost purely out of aluminum.
And the conclusion, again, is "Therefore M contains twice as many cans as L." In other words, if there were 100 cans in group L, there now must be 200 cans in group M. If there were 500 cans in group L, there are 1000 cans in group M. What do you think?
I think it's bullshit. How do we know that 100% of the material from group L was actually salvageable? Aren't there some losses involved in the recycling process? If new cans are going to be 50% recycled material, it seems far-fetched to assume that 1000 old cans will provide enough usable aluminum to make 500 new cans--that can only be true if there is zero loss in the recycling process. It seems far more likely that 1000 cans turns into enough salvage to make 400 new cans, or 300 new cans, or 200 new cans. Depending how much loss there is, isn't it possible that group M contains 10 times as many cans, or 100 times as many cans as group L? "WHAT ABOUT LOSS IN THE RECYCLING PROCESS?" That is the question I'd ask the soft drink executive on cross examination.
The question then asks "The conclusion of the argument follows logically if which one of the following is assumed?" In other words, what additional premise would prove the soft drink executive's argument? Okay, no problem. What's happening here is that we're switching teams. The soft drink executive might still be evil, but he is going to pay us a lot of money to defend his sorry ass. His argument is bad, but we're going to go dig up some evidence (perhaps we'll purchase an expert witness!) who will fill in the blanks.
I think the correct answer needs to be related to the hole in the argument that we've already identified. On cross examination, I wanted to ask about loss. So, what if we found an expert witness who could testify that there is zero loss in the recycling process? This is almost certainly untrue in real life, but we're not in real life--we're in court. If we can get a believable witness to testify that there is zero loss in the recycling process, then I think the executive's argument makes sense. That's my predicted answer: "There is zero loss in the recycling process."
A) Sorry, this is completely irrelevant. The executive was trying to prove that M had twice as many cans as L, since M is made from 50% recycled material from L. What happens after M can't possibly help this argument. No way.
B) "Quality" of recycled vs. unrecycled aluminum isn't the issue either. If you picked an answer like B, you're not focusing enough on the conclusion of the argument. How could this possibly be relevant?
C) "All of the aluminum is recovered" means exactly the same thing as "there is zero loss." This is precisely what we were looking for. If this is true, then our evil soft drink executive's argument is much stronger. This answer is the evidence we need--we need to hire an expert to say exactly this.
D) It's not relevant whether L was made from recycled cans or not. What's relevant is how many cans were in group L and how many cans were in group M. This can't be it.
E) It's not about "ease" of recycling aluminum vs. other materials. No way. C is exactly what we were looking for, so C is our answer.