Q20 - Until about 400 million

[bramon.elizabeth]

Hi! Could someone please help talk me out of answer (E)? I'm very partial to it because of the first sentence. Classic pitfall, I know, but here's my thinking:

Bob turns to Joe and says, "Until four years ago, I was a banker!" Smarty pants Joe surmises that Bob is no longer a banker.

LSAT turns to me and says, "Until about 400 million years ago, fish had characteristic X!" Doesn't that mean fish are now X-less, and have been for roughly 400 million years?

Thanks:)

[a3friedm]

Hi Elizabeth,

I really liked your analogy but I think we should look at it a little bit differently.

Until four years ago Bob and his coworkers were bank tellers and could only deal with transactions at the bank counter, Then a fast track program for a management became available. The fast track program allowed them advantages to make more money. The fast tract program diverged into two tracks, a hedge fund manager or board of director. The board of directors is more diverse.

just because the fast track program developed, doesn't guarantee us that Bob and his coworkers participated. Likewise, we can't guarantee that every fish developed jaws.

We can guarantee that before the development of jaws, the fish didn't prey upon the other fish because it is explicitly stated "Their feeding methods were limited to either sucking in surface plankton or sucking in food particles from bottom mud."

In a must be true question it is important to remember that it cannot be false. Perhaps jawless fish did become extinct 400 million years ago, however it's also possible that one species of fish didnt. That possibility makes us unable to pick (E).


Hope this helped

[ohthatpatrick]

Awesome explanation!

The problem with using the Bob analogy is that Bob is speaking about himself and no one else. The first sentence of Q20 is about a group of things.

If we say, "until 1945, Major League Baseball players were all white", we can only infer that "as of 1945, they were not all white."

i.e. we can only infer that SOME players post-1945 are non-white

Similarly, we can only infer that as of 400 million years ago, SOME fish now have jaws.

=== other answers ====

(A) 'primary' and 'all' are too extreme to be supportable

(B) we know from the 2nd sentence that jawless fish could only suck in plankton or food particles. we know from the 3rd sentence that developing jaws changed fish's life dramatically; "jaws allowed them actively to pursue prey". Supportable!

(C) We don't have enough info on 'teleosts' to make such a sweeping claim that none of them feed upon particles in bottom mud.

(D) If anything, the passage implies the opposite of this answer. As it speaks of the jawed fishes developing two separate lines, it says that "one retained cartilage for its skeletons", implying that cartilage was the pre-existing cartilage material.

[ahaghgoo]

I'm having a hard time placing the three sentences into their respective categories as the sufficient and necessary terms of their respective statements. I incorrectly considered that jawless fishes were the sufficient term and the necessary term was the feeding method of plankton or food particles.

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~J → P or FP

Then for the following statement I had fishes with jaws preyed on other fish.

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J → Prey

Taking the contrapositive of the first statement and combining it with the second statement I get:

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~P & ~FP → J → Prey

Or going the other route by taking the contrapositive of the second statement and combining it with the first I get:

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~Prey → ~J → P or FP

And the correct answer the way that I understood it, meant "if jawless fish then not preying on other fish," which I diagrammed it out to be:

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~J → ~Prey

So it's obvious that I incorrectly reversed the terms, but I don't understand how the terms are sufficient rather than my incorrect assumption of them being necessary and vice versa for the necessary terms misinterpreted as sufficient.

I'd greatly appreciate anyone's assistance.