Logical reasoning PrepTest 110 · Section 3 · Question 3

Incorrect. What will be true in the future isn't necessary for an argument about what is true today.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
S. disease is bad for bananas and the treatment is bad for humans. The treatment can't be used at small plantations but it can be used at large ones. Therefore most of the world's bananas are safe from S. disease.

Answer Anticipation:
One great way to look for errors in reasoning in an argument is to start with the conclusion identify key concepts and "work backwards" to see if those concepts are supported by the premises.

Here the key concept in the conclusion is that "most" bananas are "not seriously threatened" by S. disease. Seeing that we'd check the premises to see if the stimulus discusses "most" of the banana crop and to see what would prevent them from being threatened.

For the latter there's a fungicide that can be used to control the disease but it can only be used on large banana plantations. As such the conclusion is essentially saying that most of the world's banana crops can be treated with this fungicide and thus are grown on large banana plantations. Is that established by the premises?

It is not. While the argument does establish that there are large and small banana plantations and the large ones are definitely each larger than the small ones there may be significantly more small ones and thus they may account for most of the world's bananas. The argument is assuming that most of the world's bananas aren't on these small plantations but are rather on the large ones and can be treated with the fungicide. Let's find that answer!

Answer Explanation:
This answer establishes that most of the world's bananas are on large plantations and thus can be treated with the fungicide and therefore are not seriously threatened by S. disease. If large plantations produce only a minority of the world's bananas then the others are on plantations where the use of the fungicide is inappropriate and they're at risk for S. disease completely undermining the conclusion. This answer is necessary for the argument to hold together.

Key Takeaway:
Strengthen with Necessary Premise questions frequently bring in new concepts in the conclusion that need to be justified by attaching them to the premises. In this case the new concept was "most bananas" whereas the stimulus never used quantifiers in splitting the subsets of bananas up.

Also, working backwards from the conclusion to see which concepts are supported and what the premises say about them can be an easier way to track through the logic of the argument!

Incorrect. The speed of the spread is immaterial, as long as the disease can be treated on large plantations and not on small ones.

Incorrect. If the conclusion were about diseases in general, then this answer might be required. However, since the argument is about only the threat to bananas of S. disease, other diseases are out of scope.

Incorrect. What needs to be true for the argument to work is that most plants in the world either are on large plantations or haven't been exposed to the disease. It doesn't matter how the latter are spread out—as this answer states—as long as the majority of plants fall into one of the two categories.