LSAT Logic Games: Must Be True Questions & Minimums/Maximums

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LSAT “Which of the following must be true…” questions have a couple tricks, but after some practice can be pretty straightforward. It is crucial that you know how to master these quest
 

  • Eliminate the Opposites

    The first trick to these questions is approaching them in an elimination oriented fashion, just like with “rule testers”, meaning you should try to eliminate four answers that are wrong rather than pick the one answer that is right. The easiest way to do this is to look for answers that satisfy the opposite of what the question is asking you for. Take the opposite of “must be true” – “could be false” – and look for what four answer choices could be false. If an answer could be false, then it must not have to be true, and you know that answer can be eliminated.
    Even though “could be false” is not a perfect “opposite” of “must be true”, but there is a pretty easy formula to remember for these types of question. “Must” switches with “could be,” and “true” switches with “false.”  For example, if the question asks you for an answer that “must be false” then you should look for answers that “could be true” and you can eliminate those.

  • Reuse Your Previous Work

    Back to “must be true.” Once you have determined your opposite- “could be false”- then you can approach the possible answer choices in a couple of ways. The first, and simplest way to look for answers that could be false, is to look back at your previous work. Say you have an answer sequence from an earlier question in that game that you know is correct (maybe you know this from your first “rule tester” sequence) that goes something like A, B, C, D, E. If you have an answer that says: “B must always be after D,” then from this previous sequence you know that answer could be false as B was before D. Thus, that answer choice does not always have to be true and you can eliminate it.

  • Try Out the Remaining Answers

    If you do not have a lot of previous sequences to work from, then try testing out each answer (his takes more time, so try to use any resources you have from your diagram or rules prior to taking this step). “Testing out” simply means plugging the possible answer into your diagram to see if you can prove it doesn’t have to be true. For instance, if the possible answer choice says: “E always has to be in one of the first three slots,” try plugging E into the forth slot and see if you can come up with a sequence that does not violate any of the rules. If you can, then E could be in the forth slot and the possible answer choice could be false.


Minimums and Maximums


Logic Games questions that contain the words “what/how many” will typically be asking you to find the maximum or minimum number of spots that an activity can be in. These questions can take a little bit of time, because unlike regular Logic Games  questions, you aren’t looking for one correct sequence, but how many possible sequences there could be for a given activity.
The best way to approach these questions is to work backwards. What I mean by that is, if you are asked to find the maximum number of places an activity can be, and your answers are 1-5, see first if that activity can be in all five places. If the activity cannot be in all five places, you can then cross that answer off. Next see if the activity can be in four places, three places and so on.
This process, of eliminating the larger numbers, will help you to get through the question quickly. Working forward on these questions takes more time as you have to try out a bunch of sequences to see if the activity could be in 1 spot, then 2 spots and so forth.
As with most Logic Games  questions, the best place to start with these types of questions is to look back at your rules and see if those limit your answer choices. Then, work off your previous sequences to see if that helps. Finally, if you are still stuck between a couple of answers test out those answers and see which one leads to an incorrect sequence.

Maximum Practice Question


A student will perform six activities: L, M, N, O, P, and Q. Each activity will be performed once, one at a time. The order in which the activities are performed is subject to the following constraints:

  • L is immediately before P
  • N is sometime after Q
  • M cannot be last
  • O is either immediately after or immediately before M

If L is performed fourth, at how many different times could M be performed?
A) 1
B) 2
C) 3
D) 4
E) 5


Answer and Analysis


If L is performed forth, then from Rule 1, we know that P must be performed fifth. Right off the bat then, we know that M cannot be performed at 5 different times because only spots 1, 2, 3 and 6 are open. We can thus eliminate E.
Taking our first step of looking back at our Rules/ initial diagram, we see that because of Rule 3, M cannot be in spot 6. Thus, there are now only three possible spots that M can be, so we can eliminate answer D.
We now are down to three answer choices. From here, you may be able to look at past choices and see that M could be in any of those three spots when N is in spot 4. However, if you don’t have that work, which you might not, go on and test out the possible sequences.
If you put M in spot 1, 2, and 3, you will find that you can generate correct sequences each time (M, O, Q, L, P, N; Q, M, O, L , P, N; Q, O, M, L, P, N). Thus the correct answer is C! Hopefully, from this example you can see how it is beneficial to work backwards and eliminate the larger numbers rather than trying to work forwards.
Previous: Logic Games: Acceptability and New-If Questions
Next: What’s Tested on the LSAT: Logical Reasoning

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