PrepTest 154

Incorrect. This isn't sufficient. The argument already allows for the possibility that this is false by qualifying its conclusion with "If this is true . . . " so we don't need it to be true for the argument to hold up.

Incorrect. This alone isn't sufficient to validate the argument. Again, the argument was qualified with "if this is true," in other words, if there really is no Methane. So we're only concerned with the logic in a world where P253 really does have no Methane.

Incorrect. This doesn't prove the conclusion. We don't need to be sure there has never been life on P253 to be certain there is no life now, and knowing there was never Methane on P253 doesn't help us if we aren't sure that a lack of microbes means a lack of life.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Sufficient Premise

Stimulus Summary:
Current evidence says there's no methane on P253. If so, there's no life on P253, because: Microbes ==> Methane, and NOT Methane ==> NOT Microbes.
Answer Anticipation:
The stimulus is enough to conclude there are no microbes on P253, but why should that mean there's no life on P253? Microbes aren't the only possible form of life, are they? The missing connection here is between NOT microbes and NOT life. An answer choice that boils down to NOT Microbes ==> NOT Life or Life ==> Microbes will do the trick.

Answer Explanation:
NOT Microbes ==> NOT Life, exactly what we were looking for. This gives us the missing premise of our transitive stack.

Key Takeaway:
The concepts of "sufficient" and "necessary" are quite challenging for many students, mostly because they feel like some weird, new way of thinking that's unique to the LSAT. But look around the real world, and you'll find that sufficient and necessary conditions are all around us. Will you go to the beach if it's raining? If not, good weather is a necessary condition for you to visit the beach. Do you always get the same ice cream after having a bad day? If so, a bad day is a sufficient condition for you to buy your favorite ice cream. This question is the last in its section, and the way it's worded makes it feel very challenging. But if you truly understand how sufficient and necessary conditions work, you can see through its facade and realize the logic here is actually very simple. There's no better feeling on the LSAT than knowing that they're trying to trick you by putting a bunch of big science words on top of really basic logic, seeing through it, and solving the question in 30 seconds!

Incorrect. Wrong way around! If this said "Methane is required for life," it would work. But just knowing that the presence of Methane means the presence of life doesn't help to prove a conclusion that there is NO life.