A) because decade = 10 years and the weight of the averages changes antiproportional. hence the overal average nearly doesnt change

C) i think this is clear

18.A.. averages remain the same.. as the passage says that the same rate of increase and decrease.

19. i am not sure .

E) but when fewer vessels catch more fisch, then the average of the research vessels should be greater than before. the fact that there are fewer fish now affects both surveys.

I have B and C
For the second question, D cannot be it because if the evaluation period was the issue, then the problem would also have been present 10 years ago and the discrepancy would then not be explained.
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Best Regards,

Paul

A.
Say, 10 years ago, i.e. a decade ago the commercial catch was 20 and Survey catch was 15, the estimate would have been 20+15/2.
Based on the stem, last year, the end of the decade, the numbers would have been 30 and 5 respectively and the estimate 30 + 5 / 2.
I was initially considering B, but based on the stem B may or may not be true.

C.

All other things being equal, this choice gives a reason for the increased catch by commercial guys.

This is where the key lies for prep_gmat's explanation
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Best Regards,

Paul

This is the key to your question


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Best Regards,

Paul

Hold on guys, "C" for #2...how can we explain the number decreasing for the survey vessels, yes we can explain abt the commercial vessel, but stem says that number decreases by the same amt for survey vessels, how can we explain this using C....don't see how C can be the OA ? Anyone ?

rthothad, here is the answer to your doubts

Only commercial vessels are affected while research vessels are not.
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Best Regards,

Paul

(18)

(A) Last year’s official estimate was probably not much different from the official estimate ten years ago.
- A it is. Last year's official estimate would see the marked difference between the two estimates, as it would have been observed over the past ten years ('for the last decade....')

(B) The number of commercial vessels fishing for cod in the Grand Banks has increased substantially over the past decade.
- We can't tell if the number of commercial vessels are increasing, all we know is that the tons of cod caught per fishing effort is increasing. B is out.

(C) The sampling-based estimate is more accurate than the estimate based on commercial tonnage in that the data on which it relies is less likely to be inaccurate.
- Out of scope. No information about accuracy.C is out.

(D) The once-yearly sampling by research vessels should be used as the sole basis for arriving at the official estimate of the stock of cod.
- Again, an issue of accuracy. D is out.

(E) Twenty years ago, the overall stock of cod in the Grand Banks was officially estimated to be much larger than it is estimated to be today.
- We don't know if th everall stock of cod in the banks are much larger in the past than it is today. We only know there is a marked difference in the two estimates.

(19)

(A) Fishing vessels often exceed their fishing quotas for cod and therefore often underreport the number of tons of cod that they catch.
- Not sure how this would explain the marked difference between the two estimates.

(B) More survey vessels are now involved in the yearly sampling effort than were involved 10 years ago.
- If so, it would only improve the accuracy of the sampling-based estimate and tells us nothing about the discrepancy

(C) Improvements in technology over the last 10 years have allowed commercial fishing vessels to locate and catch large schools of cod more easily.
- Tells us more fish is caught which explained the increase in commerical estimate, and therefore the decrease in sampling estimate.

D) Survey vessels count only those cod caught during a 30-day survey period, whereas commercial dishing vessels report all cod caught during the course of a year.
- Only highlights the accuracy of the data and nothing about the difference in estimates.

(E) Because of past overfishing of cod, fewer fishing vessels now catch the maximum tonnage of cod each vessel is allowed by law to catch.
- Tells us nothing except more fishing vessels are now catching less than the maximum tonnage of cod. It still doesn't explain the discrepancy and in fact this choice should be out because we know that the commercial estimate is rising and this should be due to more cod being caught.


(A) and (C) for me.