LSAT 137 | Section 3 | Logical Reasoning: Q11

LR Question 11 Explanation

QUESTION TEXT: A mass of “black water” containing noxious organic…

QUESTION TYPE: Necessary Assumption

CONCLUSION: The black water hasn’t struck the bay with the same intensity in the past 200 years.

REASONING: Last year, the black water wiped out some coral species in Laurel Bay entirely, including some individual coral that had been there for over 200 years.

(This means: coral species X was wiped out. Some individual specimens of species X in the bay were over 200 years old.)

ANALYSIS: The author is essentially arguing:

1. The black water at this intensity eliminated 100% of some coral species.
2. Some members of those species were over 200 years old.
3. Implicit premise: Black water at this intensity would always kill 100% of those coral species.
4. Therefore, the black water had not entered the bay at this intensity for at least 200 years. Otherwise the coral mounds would have died.

The implicit premise is the weak point (and the necessary assumption). The author is assuming black water at this intensity always kills coral. But what if the coral was stronger in the past?

___________

1. The argument isn’t about how often black water enters the bay. It’s about how often black water enters the bay at that intensity.
    
   If a weak mass of black water entered the bay 150 years ago, that has no impact on the argument. 
2. This isn’t necessary. The author was only arguing that five species of coral were wiped out. The mounds could have been members of one of the five species that were particularly vulnerable to black water.
3. The argument is only about coral. We’re not concerned with the effects on other species.
4. CORRECT. This negation wrecks the argument.
   Negation: The coral was especially vulnerable last year when the black water arrived.
5. It doesn’t matter whether old or young coral was more vulnerable: everything died.