Complete Question Explanation, compiled in part from instructor responses below.

This question ranks as one of the hardest LSAT questions ever, with only around 13% of all test takers selecting the correct answer choice.

This is all of the information we have about cod in the Grand Banks:

1. We estimate how much cod is available by averaging two estimates, which I'll call (X) and (Y).

2. (X) is based on the number of cod (apparently the total number) caught by research vessels at such-and-such time.

3. (Y) is based on the average number of tons of cod caught by commercial vessels per unit of "fishing effort."

4. (X) and (Y) have usually been pretty close to each other, but ...

5. ... lately, (Y) has been going up, and (X) has been going down at about the same rate.

Here's another example:

Answer choice (B) is incorrect since the commercial fishing figure is simply an average of the "number of tons of cod caught by various commercial vessels per unit of fishing effort." Since the commercial figure is an average, this gives us no indication of whether the number of commercial vessels has increased.

The problem with answer choice (B) is that we haven't the faintest idea about the size of the fishing fleet in the Grand Banks, and whether it's increased, decreased, or stayed the same over time. We know that the estimate of the cod stock based on commercial fishing has increased, but we don't even know anything about what's caused it to do so, much less about what that might tell us about fishing vessel count (all it means is that the amount of cod caught "per unit of fishing effort" has gone up, and that's it). So attempting to conclude anything about how many commercial boats in the Grand Banks are now fishing for cod compared to the past decade, especially saying it "increased substantially," is entirely impossible and (B) is gone as a result.

Answer Choice (C) is incorrect because there is no information given in the stimulus by which to draw that inference. The stimulus does not give any information on the accuracy of the two different methods. Since this is a must be true question we are looking for an inference that can be drawn from the information given in the stimulus.

Since the estimate is taken by averaging the two different measurements, and one has gone up over the last ten years by roughly the same amount that the other measurement went up, we know that the most recent estimate should be fairly close to the one that was taken ten years ago because the change in the two measurements will average each other out.

Answer choice (E) is incorrect. (E) is about 20 years ago—so there's simply no way to know any details about this unmentioned period in time and what the cod situation was like back then, and so we definitely do not know how much cod we had twenty years ago. All we know is that the two methods for estimating that amount used to be close to one another, and in the last decade they've been moving in different directions at about the same rate.

For example, let's say that 10 years ago, both the estimate from the research vessels and the estimate from the commercial fishing vessels totaled 10 units. The average of 10 and 10, then, is 10. Ten years later, the research vessel estimate is decreasing so let's say it's now at 5 units. But the fishing vessel estimate is increasing by about the same amount so let's say it's now at 15 units. The average of 5 and 15 is still 10.

Therefore, if both estimates used to be about the same and now one is increasing at about the same rate as the other is decreasing, the average of those two estimates is probably still about the same as it was 10 years ago. This leads us to answer choice (A).

**Must Be True. The correct answer choice is (A)**This question ranks as one of the hardest LSAT questions ever, with only around 13% of all test takers selecting the correct answer choice.

This is all of the information we have about cod in the Grand Banks:

1. We estimate how much cod is available by averaging two estimates, which I'll call (X) and (Y).

2. (X) is based on the number of cod (apparently the total number) caught by research vessels at such-and-such time.

3. (Y) is based on the average number of tons of cod caught by commercial vessels per unit of "fishing effort."

4. (X) and (Y) have usually been pretty close to each other, but ...

5. ... lately, (Y) has been going up, and (X) has been going down at about the same rate.

**Answer choice (A): this is the correct answer choice.**Consider that the estimate is based on an average of two separate figures. Over the last ten years, one of the figures has been rising and the other has been falling, both at about the same rate. Essentially, the rise in one figure is offset by the drop in the other figure and so the estimate remains approximately the same. For example, say that in the first year both figures equaled 10 units each, and thus when averaged the estimate would be 10 units. Ten years later one figure has risen to 15 units while the other has dropped to 5 units. When averaged, these two figures also produce an average of 10 units.Here's another example:

- Traditionally both (X) and (Y) = 50.

But for the last ten years, (X) has been dropping by 2 per year, while (Y) has been increasing by 2 per year. So our numbers have diverged, moving from 50/50 to:

48/52, then

46/54, then

44/56, then

42/58, then

40/60, and so on.

But the AVERAGE has remained the same, right? It's always 50, because (X) and (Y) mirror each other, moving by the same amount in opposite directions.

Answer choice (B) is incorrect since the commercial fishing figure is simply an average of the "number of tons of cod caught by various commercial vessels per unit of fishing effort." Since the commercial figure is an average, this gives us no indication of whether the number of commercial vessels has increased.

The problem with answer choice (B) is that we haven't the faintest idea about the size of the fishing fleet in the Grand Banks, and whether it's increased, decreased, or stayed the same over time. We know that the estimate of the cod stock based on commercial fishing has increased, but we don't even know anything about what's caused it to do so, much less about what that might tell us about fishing vessel count (all it means is that the amount of cod caught "per unit of fishing effort" has gone up, and that's it). So attempting to conclude anything about how many commercial boats in the Grand Banks are now fishing for cod compared to the past decade, especially saying it "increased substantially," is entirely impossible and (B) is gone as a result.

Answer Choice (C) is incorrect because there is no information given in the stimulus by which to draw that inference. The stimulus does not give any information on the accuracy of the two different methods. Since this is a must be true question we are looking for an inference that can be drawn from the information given in the stimulus.

Since the estimate is taken by averaging the two different measurements, and one has gone up over the last ten years by roughly the same amount that the other measurement went up, we know that the most recent estimate should be fairly close to the one that was taken ten years ago because the change in the two measurements will average each other out.

Answer choice (E) is incorrect. (E) is about 20 years ago—so there's simply no way to know any details about this unmentioned period in time and what the cod situation was like back then, and so we definitely do not know how much cod we had twenty years ago. All we know is that the two methods for estimating that amount used to be close to one another, and in the last decade they've been moving in different directions at about the same rate.

For example, let's say that 10 years ago, both the estimate from the research vessels and the estimate from the commercial fishing vessels totaled 10 units. The average of 10 and 10, then, is 10. Ten years later, the research vessel estimate is decreasing so let's say it's now at 5 units. But the fishing vessel estimate is increasing by about the same amount so let's say it's now at 15 units. The average of 5 and 15 is still 10.

Therefore, if both estimates used to be about the same and now one is increasing at about the same rate as the other is decreasing, the average of those two estimates is probably still about the same as it was 10 years ago. This leads us to answer choice (A).