Eliminating CNBTs in CBT Qs

    • August 15, 2017 at 7:01 pm #3270

      In the Trainer, Mike suggests employing process of elimination for CBT Qs and looking for MBT/CNBT and moving on immediately after finding the MBT/CNBT in MBT/CNBT Qs.

      But sometimes when the right answer of a CBT Q is (A) or (B), or when the ACs are designed in a way that makes it harder to see if each AC is CNBT (whether it’s because of the AC is really wordy or because CNBT ACs don’t immediately jump out like they do in other Qs), eliminating 4 CNBT ACs seems counterintuitive.

      So at this point, I am a bit puzzled in terms of my Q type strategy for logic games. While it’s incredibly helpful that I actively, consciously look for CNBT ACs to eliminate in CBT Qs, elimination is inefficient in some CBTs. Does anyone have any advice for me?

    • August 15, 2017 at 11:09 pm #3271
      LSAT Dan

      I think if you keyed your question here to a specific past PT question or two, I could address it more helpfully. Let me know if you have an example of one or two that fit.

    • August 16, 2017 at 9:29 am #3272

      Hi Dan,

      I think both PT34 S4 G1 Q5 and PT56 S1 G1 Q5 are the examples of the CBT Qs where picking the right answer directly instead of eliminating CNBT answer choices is the optimal approach. I’d really appreciate your insight on these Qs!

    • August 16, 2017 at 3:00 pm #3273
      LSAT Dan

      Typically, on a CBT question, you want to find a layout where the answer choice is true; if you find one example, you’re gold. Sometimes, the rules will eliminate the other answer choices, the “MBF” (Must Be False, which I assume is what you mean by CNBT (can not be true?)). It’s usually the case, though, that the reasons that the MBF answers are wrong is somewhat hidden (except on acceptability questions), so generally, I’d be looking for layouts that show that one of the answer choices is possible, although I might scan the choices first and see if I can easily eliminate them (maybe all 4, but not usually).

      As for the questions you asked about, the first thing I’d do on the first one is address that ambiguity about “the maximum number of aisles that could separate them,” and as is usually the case, the efficient way to address that is systematically. The theoretical maximum is to put her aisles on the endpoints, but that’s impossible, because of the L rule. So can I move one of them in by just one spot? I can’t give her aisle 2, because that’s Kurt’s aisle. That leaves aisles 1 and 8 for Larissa, and I’d plug that into my diagram:


      Actually, I’d work this game with templates relying on the limited possibilities for the MKM block. Because we have to leave room for O and L to follow it, that block can only occupy positions 345, 456, or 567. But let’s say I weren’t using templates. Having placed L and positions 1 and 8, there are now only two places for MKM – 345 or 456 (567 is gone, because there’s no room for the other O, which much come before position 8. Since there are only two possibilities, and each one takes care of half of my remaining variables, I’d plug them both in to examine them on separate tracks. It’s got to be either:


      In the bottom one, all that’s missing are two Js, so we can fill them in. In the top one, we’re missing two Js and the O, and the J’s can’t be consecutive, so one of the is in position 9. So it’s:

      LKMKM J/O O/J L J

      The first of these layouts matches (A), so we’re done. Notice that the bottom layout also eliminates (A) and (D) on the next question, so I’ve already found two of the wrong answers. I’m always looking to use past layouts to help me on future questions.

      Question 5 on PT 56 is more wide-open, so I’d base my work on the answer choices more than the initial setup (there are too many places F could be). So I’d start with (A):

      The first rule is already satisfied; wherever we put J, it will come before H. We have:

      GL – K
      and J and L are separated by 1.
      And F can’t be first.

      Again, to be systematic, there are only two possibilities – G is first, or L is first. I’d pick one of them and try it. If G is first, L is second, and J is 4th (JL rule):

      GL?J?H F and K are interchangeable at this point, so I throw one in there:

      I run down my list of rules: J is before H; G is before K; G and L are consecutive; J and L are separated by 1 audition. So (A) is right -If F is not at 1 PM, H COULD BE at 6. Once you find one that works, you’re done.

      Sometimes, it’s not hard to pick off the wrong answers on a CBT, but other times, it can be. Usually, I’d spot check the answers, but if I’m not seeing clear, quick, easy ways to eliminate answers, I’m not going to do the deer in the headlights thing; I’m going to grab an answer choice and run with it.

    • August 17, 2017 at 6:58 am #3274

      Thank you so much for your help, Dan.
      Can I ask you what is your perspective (like what you think when you first read the Q stem) in regards to approaching a CBT Q?
      I think what really helped me was deliberately looking for MBF ACs in CBT Qs. While you said ‘Typically, on a CBT question, you want to find a layout where the answer choice is true,’ isn’t the logic game (and the LSAT as a whole) really about what MUST BE TRUE? The critical deductions of the games are about what must be true and cannot be true. Also, it’s harder and takes longer to think about what can be true, which is why I believe eliminating MBF choices is actually more efficient in CBT Qs. I’d like to know how you think!

    • August 17, 2017 at 11:09 pm #3276
      LSAT Dan

      When certain layouts are not possible (which is why certain things must be either true or false), broadly speaking, there are two possibilities – either something will have to be false because it explicitly violates a single rule, or possibly an obvious inference from a couple of rules combined; or, alternatively, an invalid layout will be invalid because of the interaction of multiple rules in a way that’s not readily apparent.

      Often, you can tell which is more likely to be the case on a particular game – if there is a bunch of available inferences up front, it’s more likely you can deduce things that can tell you right away that certain answer choices must be false. Also, on acceptability questions, you’re pretty much always looking for the MBF answers by running the rules, and eliminating wrong answers, using a process of elimination approach.

      But on a lot of questions, it won’t be that clear why the wrong answers must be false, in which case you’re typically testing answer choices. When that happens, it doesn’t really matter if you say you’re looking for a CBT or a MBF. You’re just going to test it, and you’re going to determine which is the case – if you can make it work, it’s your answer; if it won’t work, then it MBF and you can eliminate it.

      I wouldn’t say that the game is “about” what Must Be True; the rules are there to prohibit certain layouts, and may questions just want you to figure out what’s possible. It all depends on the question.

      The real crux of the issue, I would say, is this – If there are easy inferences that allow you to eliminate an answer, you want to find them, but very often, students spin their wheels looking for quick inferences that aren’t there, when they’d really be better off just grabbing an answer choice and proving it right or wrong, and on a CBT question, you do that by starting with the assumption that an answer choice is true – if you can generate a complete layout around it that doesn’t break any rules, that’s your answer, and if not, then it MBF and it’s a wrong answer. You don’t want to miss the easy inferences, but you also don’t want to burn a lot of time looking for inferences that aren’t there. So typically, I’ll adopt a compromise approach – I’ll spot check the answer choices and see which, if any, I can eliminate at not much more than a glance. But I’m not going to spend more than about 5-10 seconds per answer choice; if that doesn’t answer the question, I’m brute forcing it. I’m also (before I do anything, actually), looking at my past layouts – often, I’ll have one that tells me that one of the answer choices could be true, in which case the question is over before it starts.

      Hope this helps, and don’t hesitate to post about any particular game or question.

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