Logical reasoning PrepTest 146 · Section 3 · Question 5

Incorrect. Since the premises are all about percentages, this answer about the number of houses is incorrect.

Incorrect. This answer is trying to get you to think it's saying that the two sets overlap by connecting inadequate site drainage to being unsafe. However, the second group was unsafe because of structural defects, and this answer doesn't establish that the inadequate site drainage caused a structural defect.

Incorrect. The difficulty of fixing any of these issues is out of scope since the argument doesn't discuss unfixable or unlikely to be fixed problems.

Incorrect. Since the argument concludes that at least 60% of the houses have some kind of problem, it leaves open the possibility that many houses—up to 40% of them—don't have either of these issues.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
In a town: 30% of houses have bad drainage. 30% of houses have structural defects.Therefore: At least 60% of houses have a major problem.

Answer Anticipation:
Whenever the LSAT tries to add percentages together, shake your finger at it and say, "Not so fast, LSAT." (Well, do that quietly.)

In order for these percentages to add together, the houses in the two sets must be unique—in other words, there can't be overlap. If most of the houses with poor drainage also have structural defects, then that house would count in both percentages and thus adding them together would double-count that house.

This is, in essence, an overlapping set flaw—the Resident treats these two sets as not overlapping at all, so he ignores the possibility that some houses belong to both groups.

Answer Explanation:
This answer highlights that there might be shared members of the groups and thus the percentages can't just be added together.

Key Takeaway:
When percentages are added together on the LSAT, get suspicious. There's usually a chance that the elements in each group could overlap, and thus simply adding those percentages together double-counts them.