#13 - Standard aluminum soft-drink cans do not vary in...

[Administrator]

Complete Question Explanation

Question #13: Justify. The correct answer choice is (C).

The argument is structured as follows:

• Premise: 50% of the aluminum in M was recycled from another group (L) of used aluminum cans.
  Premise: All the cans in L were recycled into cans in M (L M).
  Premise: Aluminum cans don't contain much of anything else, other than aluminum, and don't vary in the amount of aluminum that they contain.
  
  Conclusion: M contains twice as many cans as L (M = 2*L)

At first glance, the argument seems reasonable. Let's say there are 100 used cans (L = 100), all of which were recycled into M cans. However, only half of the aluminum in the M cans came from L: the other half must have come from somewhere else. Clearly, then, assuming that the cans don't vary in the amount of aluminum they contain, and don't contain any other materials, the M group must be twice as big as the L group (M = 200).

One piece is missing for this argument to be bullet-proof: we need to assume that there is no loss of aluminum during recycling. If there was, and the 100 L cans provided 50% of the aluminum in M, then the M group would be less than twice the size of the L group. This prephrase agrees with answer choice (C). Although this is a Justify question, answer choice (C) is both sufficient, and necessary, for the conclusion to be logically valid. In other words, answer choice (C) would have been also correct if this were an Assumption question.

Answer choice (A) is incorrect, because whether the aluminum in M can be further recycled or not has no bearing on the conclusion of the argument.

Answer choice (B) is incorrect, because the quality of the aluminum used is entirely irrelevant to this argument.

Answer choice (C) is the correct answer choice, as explained above.

Answer choice (D) is incorrect, because the source of aluminum in L has no bearing on its ability to be recycled into M.

Answer choice (E) is incorrect, because it is entirely outside the scope of the argument. How aluminum compares to other materials in terms of recycling has no bearing on this conclusion.

[lsatfighter]

When I first did this question, I thought to myself, "M is composed of 50% aluminum from L and 50% aluminum from an unknown source. As for L's composition, L gave about 50% of its aluminum to M, but what happened to the other 50% of L's aluminum? It must have been disposed of and rendered useless after the whole recycling process due to its decreased quality. So B seems to describe that. C describes the opposite of my thinking. So B must be right and C must be wrong."

I can now see that, if the other 50% of L's aluminum was disposed of, then we wouldn't be able to explain why M contains twice as many cans as L (as stated in the conclusion). So here's my current analysis of this whole question:

L gave about 50% of its aluminum to 1 M can and the other 50% of L's aluminum went to a second M can. Therefore, for every L can, there were 2 M cans made. This supports the conclusion. All of L's aluminum was recovered and used to make M cans, making C the correct answer. B is wrong, because it's out of scope. Quality and unrecycled aluminum is irrelevant to the stimulus. We don't know if L is recycled or unrecycled. We also don't know if L is of better or poorer quality than M.

Does my current analysis of this question seem fine to you? Did I miss anything? Is there any other reason why C is right and B is wrong?

[Charlie Melman]

Hi lsatfighter,

Your explanation of answer choice (B)'s incorrectness is spot-on -- nothing in the stimulus indicates we should care about the quality of the aluminum in M cans (the recycled aluminum). We just care about amounts of aluminum that have been recycled.

I mostly agree with your explanation of answer choice (C), but it might be a little confusing to say that for every L can, two M cans are made. If all L cans go into M cans, and M cans are made of 50% L cans, then one L can gives 100% of its aluminum to one M can, and that M can is made of 50% L can aluminum and 50% other aluminum. The reason why answer choice (C) is right is that the above numbers only work if the L can is able to give 100% of its aluminum to an M can. Say we can only harvest 10% of the aluminum in L cans instead of 100%. Now it takes 10 times as many L cans to contribute the same amount of aluminum to M cans.

The key is that the stimulus says "50% of the aluminum" in M cans comes from L cans -- not that there are 50% as many L cans as M cans. Answer choice (C) bridges the gap between "amount of aluminum" and "number of cans."

Hope this helps! This is a tough question.

[LogicalReasoning]

I understand answer choice D to mean recycled aluminum: of used, standard aluminum soft drink cans (as referenced in the question to group M cans). Therefore answer choice D would be the BEST answer.

Why is it that we cannot assume this since this assumption is definitely plausible and also compatible with the passage?
What is meant by: "You should not make assumptions that are by
commonsense standards implausible, superfluous, or incompatible with the passage"?

Thank you

[Robert Carroll]

LogicalReasoning,

This is a Justify question, so we're looking for an answer choice that proves the conclusion. I agree that answer choice (D) is consistent with the argument, but it does nothing to prove the conclusion in any way. If the aluminum in the L cans were recycled, or not, what difference would that make? As it stands, we have a problem with the argument because we don't know that recycling aluminum preserves the amount of recovered aluminum. That the L cans were recycled doesn't help with that; that they weren't already recycled doesn't either.

It's not that we can't assume answer choice (D), but that assuming answer choice (D) doesn't prove the conclusion.

It's true that, given the instructions in Logical Reasoning, we should not make assumptions that are implausible, superfluous, or incompatible with the passage. But that doesn't mean we should make EVERY assumption that's plausible, not superfluous, or compatible with the passage. We should generally speaking not make assumptions at all unless that's the job of the question, and we should always follow the principles of what the question is asking. This question is asking for an assumption that would prove the conclusion. So we should look for such an assumption in the answers. Because answer choice (D) does nothing to prove the conclusion or even help the argument, it's not a good answer choice.

Robert Carroll

[ltowns1]

I had a question about this one that doesn’t necessarily relate to the final answer, but it got me thinking. How could this argument get to the conclusion that M has twice as many as L even if we were to assume that all the aluminum was recycled? Wouldn’t that mean that L would have 0 cans and M would have double the amount of cans that it did previously, but not double of what L had because L would now have 0 cans since it recycled all of them into M?

[Robert Carroll]

ltowns1,

There is no change in the number of cans happening here. M has twice as many as L had. M is not growing and L is not shrinking.

Robert Carroll