#15 - Local, Cannot Be True, False to True

[Dave Killoran]

Complete Question Explanation
(The complete setup for this game can be found here: lsat/viewtopic.php?t=7106)

The correct answer choice is (A)

This question asks for what must be false. Remember, always convert the question in terms of false into terms of true (false-to-true, or FTT). In this case, the condition in the question stem indicates that 1st street is cleaned earlier than 3rd street, and applying the inference we made in the setup yields that 1st street must be cleaned on Monday morning. Hence, answer choice (A), which asserts that 1st street is cleaned on Tuesday afternoon, cannot be true and is thus correct.

[jamip]

I'm working Game #3 (street cleaning) and question #15 (the answer) states that I must covert the question in terms of false into terms of true. How exactly do I go about doing this.

[Jon Denning]

Hey jamip,

Thanks for the question. This sounds like Game 3 from the February 1992 LSAT, and is an Advanced Linear game where our setup uses days of the week as the base, with a morning and afternoon spot for each day to track when streets are being cleaned. I won't go into much more detail about the specific setup since you're only asking about question 15, but I will note that it's important in situations like this to pay attention to the distribution of spaces to be filled and variables (streets, in this case) to fill them. Fortunately this is a manufactured Balanced distribution, where the 8 streets and the two empty spaces (Fri morning and Wed afternoon) account for all ten spots: 5 days with an AM and a PM each.

The nature of the question you reference is fairly common in that it is posed in terms of Falsity---specifically "must be false"---in asking you to determine the answer. Because thinking in terms of falsity is often challenging for people, we encourage you to convert from False to True, as is first covered in Lesson 2 of our course under Logical Opposition/Logical Equivalence. On page 3 of this section we show conversions between all of the phrasings using "false" and their logically equivalent "true" counterparts, and I'd encourage you to memorize these and continue to practice with them as you approach test day.

Looking at question 15 and the question "what must be false?," what you are really being asked is to determine an answer choice that "Cannot be True." That is, given the local conditions of that question, which answer scenario cannot occur. What that means for the incorrect answers is that each of them COULD potentially be true, in that they are either possible (could), or certain (must).

I think sometimes, especially early on, this type of conversion is made more clear by imagining a number line from 0 to 100, where 0 is impossible (cannot be true), and 100 is certain (must be true). This allows you to logically consider what a "false" statement would include/exclude on the number line, and then determine the equivalent "true" statement that would give the same result. Consider by that reasoning the statement "could be false." That represents everything on the number line from 0-99; literally "could be false" is everything except for 100, as it does not have to occur (remember: 100 is certain to occur). So the equivalent "true" would be "Not necessarily true": it could be true (1-99) or it could not be true (0); it simply does not HAVE to be true (100). [Of course, if that's confusing you can always just memorize the relationships in the book and use those instead ]

I hope that helps! If you have further questions please let us know!