## LR - Must be True

Prepare for the LSAT or discuss it with others in this forum.
danitt

Posts: 1983
Joined: Wed Sep 07, 2011 9:40 pm

### LR - Must be True

I have been having quite the time getting Must Be True questions down. They seem to be the ones that give me that I get wrong the most during my PTs. Can anyone give any helpful tips to really working them through? (Or maybe I'm just a dumbass Idk).

Scotusnerd

Posts: 811
Joined: Sat Oct 01, 2011 7:36 pm

### Re: LR - Must be True

Whenever I see a 'Must be true' question, I start to think of it as a connect the dots question.

Example:

Johnny has a dog. All people who have big dogs are friendly. Therefore, Johnny is friendly.

Which of the follow must be true?
A) Johnny has a poodle
B) Jimmy has a big dog
C) Johnny's cat is larger than most dogs
D) Johnny has a big dog
E) Johnny has a friendly dog

If you look at the problem, you see that there's a gap in the logic that needs to be filled in. Johnny has a dog, yes, but only people who have big dogs are friendly. So how do we know for certain that Johnny is going to be friendly?

A) Doesn't answer the question that well, since it's possible to think of a small poodle.
B) Jimmy isn't in the question
C) No one cares if Johnny has a cat
D) Credited response
E) Outside of the scope of the question. Johnny could be a mean SOB even if his dog is friendly.

If I have difficulty, I just reverse-engineer the problem. I go down the list of answers and plug the opposite in. When I reach one that would create a situation that's impossible (Johnny has a small dog), I know I've found the right answer.

Hope that helps!

danitt

Posts: 1983
Joined: Wed Sep 07, 2011 9:40 pm

### Re: LR - Must be True

Scotusnerd wrote:Whenever I see a 'Must be true' question, I start to think of it as a connect the dots question.

Example:

Johnny has a dog. All people who have big dogs are friendly. Therefore, Johnny is friendly.

Which of the follow must be true?
A) Johnny has a poodle
B) Jimmy has a big dog
C) Johnny's cat is larger than most dogs
D) Johnny has a big dog
E) Johnny has a friendly dog

If you look at the problem, you see that there's a gap in the logic that needs to be filled in. Johnny has a dog, yes, but only people who have big dogs are friendly. So how do we know for certain that Johnny is going to be friendly?

A) Doesn't answer the question that well, since it's possible to think of a small poodle.
B) Jimmy isn't in the question
C) No one cares if Johnny has a cat
D) Credited response
E) Outside of the scope of the question. Johnny could be a mean SOB even if his dog is friendly.

If I have difficulty, I just reverse-engineer the problem. I go down the list of answers and plug the opposite in. When I reach one that would create a situation that's impossible (Johnny has a small dog), I know I've found the right answer.

Hope that helps!

Why does this feel like an assumption question?

bp shinners

Posts: 3086
Joined: Wed Mar 16, 2011 7:05 pm

### Re: LR - Must be True

danitt wrote:Why does this feel like an assumption question?

Because it is.

For Must Be True questions, you're going to be looking for something that you are 100% sure of. It's a 'bet your life' kind of question - there's no gap in the premises, and you're 100% sure that your answer choice is backed up by them (as opposed to soft Must Be True questions, that have a prompt similar to, "Which one of the following is MOST strongly supported by the information above?", where there's a little wiggle room for uncertainty).

These questions are often diagrammed (a little over 50% of the time) - if you can diagram it, that will get you the answer.

If it's not diagrammable, the questions tend to push you towards a certain conclusion. They give a lot of information about, say, a study they were doing, but they don't fill in the last 'box' - the conclusion to be drawn from the information presented.

The most common way that they'll trick you in these questions is with an answer that equivocates between a term in the stimulus and another term that you consider to mean the same thing. For instance, many incorrect answers will have a stimulus that talks about fat loss, and an answer that talks about weight loss. Most people equivocate those two terms, but on the LSAT (and in the real world), they mean very different things (i.e. I can lose fat but gain weight if I'm on a Jersey Shore-level of 'roids).

danitt

Posts: 1983
Joined: Wed Sep 07, 2011 9:40 pm

### Re: LR - Must be True

The details need to be an exact match so I have to watch for detail creep. Duly noted. Thank y'all for your help. Hopefully it sticks.

JamMasterJ

Posts: 6657
Joined: Sat Jan 29, 2011 7:17 pm

### Re: LR - Must be True

Scotusnerd wrote:Whenever I see a 'Must be true' question, I start to think of it as a connect the dots question.

Example:

Johnny has a dog. All people who have big dogs are friendly. Therefore, Johnny is friendly.

Which of the follow must be true?
A) Johnny has a poodle
B) Jimmy has a big dog
C) Johnny's cat is larger than most dogs
D) Johnny has a big dog
E) Johnny has a friendly dog

If you look at the problem, you see that there's a gap in the logic that needs to be filled in. Johnny has a dog, yes, but only people who have big dogs are friendly. So how do we know for certain that Johnny is going to be friendly?

A) Doesn't answer the question that well, since it's possible to think of a small poodle.
B) Jimmy isn't in the question
C) No one cares if Johnny has a cat
D) Credited response
E) Outside of the scope of the question. Johnny could be a mean SOB even if his dog is friendly.

If I have difficulty, I just reverse-engineer the problem. I go down the list of answers and plug the opposite in. When I reach one that would create a situation that's impossible (Johnny has a small dog), I know I've found the right answer.

Hope that helps!

no, people without big dogs can be friendly, they just don't have to be.

062914123

Posts: 1846
Joined: Sun Apr 20, 2008 2:11 pm

### Re: LR - Must be True

.
Last edited by 062914123 on Sun Jun 29, 2014 6:03 pm, edited 1 time in total.

VasaVasori

Posts: 571
Joined: Sun Dec 11, 2011 2:36 pm

### .

.
Last edited by VasaVasori on Sat May 02, 2015 10:58 pm, edited 1 time in total.

bp shinners

Posts: 3086
Joined: Wed Mar 16, 2011 7:05 pm

### Re: LR - Must be True

bee wrote:and I'm pretty sure the LSAT has never tried that Jimmy/Johnny answer choice trick.

They've never been that obvious about it with a name, but they've definitely equivocated between very similar nouns. It's actually come up a couple times now that I can remember off the top of my head (radio psychiatry and practical jokes).

062914123

Posts: 1846
Joined: Sun Apr 20, 2008 2:11 pm

### Re: LR - Must be True

.
Last edited by 062914123 on Sun Jun 29, 2014 6:03 pm, edited 1 time in total.

Scotusnerd

Posts: 811
Joined: Sat Oct 01, 2011 7:36 pm

### Re: LR - Must be True

Oh they've done it all right. I remember one PT in the 50s that had an LR that had something to do with Apples and pasteurization. That question tripped me up good.

Posts: 2063
Joined: Sat Jan 16, 2010 4:40 am

### Re: LR - Must be True

danitt wrote:I have been having quite the time getting Must Be True questions down. They seem to be the ones that give me that I get wrong the most during my PTs. Can anyone give any helpful tips to really working them through? (Or maybe I'm just a dumbass Idk).

Do the following, and (as long as done properly), you will never miss another MBT question, I promise.

After eliminating two or three of the answers, use the "negation test" on the remaining answers. Reverse each answer to see if, given the stimulus information, the answer can be false. If the answer cannot be false given the stimulus information, then it MUST BE TRUE. The concept works the same way in any LSAT MUST BE TRUE question, including LG sections.

Hint: Any "inference" or "properly inferred inference" ("most strongly supported", etc.) is an answer that also MUST BE TRUE given the stimulus. Hence, you should treat all inference questions as must be true questions and check the answers the same way. Remember that the logical arrow is "downward", meaning the question denotes the effect of the stimulus on the correct answer, as opposed to the answer's affect on the stimulus (which would be an "upward" logical arrow).

Additional hint: This concept also applies to (1) MAIN POINT and (2) POINT AT ISSUE questions, as well as all REASONING question types: (3) FLAW, (4) METHOD, and (5) PARALLEL. They are all MUST BE TRUE question types in disguise. The correct answer cannot be false given the stimulus. However, the negation test is unnecessary with these five question types because the answers are "evaluative" of the stimulus, as opposed to "substantively" related.

Parallel reasoning requires that the correct answer mirror the logical structure of the stimulus, not the subject matter, content or other elements.

062914123

Posts: 1846
Joined: Sun Apr 20, 2008 2:11 pm

### Re: LR - Must be True

.
Last edited by 062914123 on Sun Jun 29, 2014 6:04 pm, edited 1 time in total.

danitt

Posts: 1983
Joined: Wed Sep 07, 2011 9:40 pm

### Re: LR - Must be True

danitt wrote:I have been having quite the time getting Must Be True questions down. They seem to be the ones that give me that I get wrong the most during my PTs. Can anyone give any helpful tips to really working them through? (Or maybe I'm just a dumbass Idk).

Do the following, and (as long as done properly), you will never miss another MBT question, I promise.

After eliminating two or three of the answers, use the "negation test" on the remaining answers. Reverse each answer to see if, given the stimulus information, the answer can be false. If the answer cannot be false given the stimulus information, then it MUST BE TRUE. The concept works the same way in any LSAT MUST BE TRUE question, including LG sections.

Hint: Any "inference" or "properly inferred inference" ("most strongly supported", etc.) is an answer that also MUST BE TRUE given the stimulus. Hence, you should treat all inference questions as must be true questions and check the answers the same way. Remember that the logical arrow is "downward", meaning the question denotes the effect of the stimulus on the correct answer, as opposed to the answer's affect on the stimulus (which would be an "upward" logical arrow).

Additional hint: This concept also applies to (1) MAIN POINT and (2) POINT AT ISSUE questions, as well as all REASONING question types: (3) FLAW, (4) METHOD, and (5) PARALLEL. They are all MUST BE TRUE question types in disguise. The correct answer cannot be false given the stimulus. However, the negation test is unnecessary with these five question types because the answers are "evaluative" of the stimulus, as opposed to "substantively" related.

Parallel reasoning requires that the correct answer mirror the logical structure of the stimulus, not the subject matter, content or other elements.

I'm literally printing this out. Thank you so much!

bp shinners

Posts: 3086
Joined: Wed Mar 16, 2011 7:05 pm

### Re: LR - Must be True

bee wrote:
Scotusnerd wrote:Oh they've done it all right. I remember one PT in the 50s that had an LR that had something to do with Apples and pasteurization. That question tripped me up good.

I remember that one... didn't seem to be quite the same concept.

Yea, that question is a MBT question that turns on the fact that the stimulus uses very strong language ('most effective' I believe) - it was more about logical force than switching nouns/scope of nouns.

And I'm going to go have a drink now to make myself feel better for knowing almost everything about that question from a vague description.

meandme

Posts: 74
Joined: Wed Feb 15, 2012 9:36 pm

### Re: LR - Must be True

Hey guys
What do you do when you can't comprehend the content of the stim in given time in a MBT or a soft MBT?

Thanks

Posts: 2063
Joined: Sat Jan 16, 2010 4:40 am

### Re: LR - Must be True

danitt wrote:I have been having quite the time getting Must Be True questions down. They seem to be the ones that give me that I get wrong the most during my PTs. Can anyone give any helpful tips to really working them through? (Or maybe I'm just a dumbass Idk).

Realize that Must Be True answers and Inferences/Proper Inferences work exactly the same way in LSAT logic, and they work that way in every section: LR, LG and RC. The correct answer cannot ever be false given the stimulus info. First, to narrow down, you must realize the types of wrong answers that are common in must be true/inference LR questions.

A bad inference is something that can be true but doesn't have to be true given the stimulus. So you must find the answer that absolutely must be true, and you can only do that by testing it.

1) New information answers. These types of answers are not inferable. If an answer gives new information, it's wrong because it's out-of-scope. A proper inference can only occur when it is within the same scope as the stimulus. Usually there are one or two of these. The stimulus might specifically discuss "college professors", whereas an out-of-scope answer might mention "all teachers".

2) Opposite or reverse answers. These are answers that, in their original, un-negated form, would directly contradict the stimulus. Cross those off. An example of such a bad inference might be "Any boy who can play sports is brainy." The proper inference might be "Any boy who can play sports is NOT brainy." The first version is an "opposite answer". Another example is "If you don't go to the fair, you will win a prize". The proper inference might be "If you don't go to the fair, you will win a prize".

3) Shell game answers are also commonly used. The incorrect inference might be "You can fool some of the people all of the time". The correct inference might be "You can fool all of the people some of the time". Switching the placements of the words "all" and "some" vastly changed the meaning(s) of the sentences. That's why it's called "shell game" answers.

After narrowing down to two answers, use the same negation method you use to test necessary assumptions. Any answer that must be true cannot be false given the stimulus info. Negate the answer and insert it at the end of passage (instead of just before the conclusion, which you would do with an assumption). If the negated answer that directly opposes the stimulus is the correct answer. It cannot be false and must therefore be true.

bp shinners

Posts: 3086
Joined: Wed Mar 16, 2011 7:05 pm

### Re: LR - Must be True

PDaddy wrote:Hint: Any "inference" or "properly inferred inference" ("most strongly supported", etc.) is an answer that also MUST BE TRUE given the stimulus.

This is incorrect. If the prompt says 'most strongly supported', then it doesn't 100% have to be true, and the negation test will fail. There's a little wiggle room in most strongly supported questions (we call the soft Must Be True at BP because of this wiggle room).

For instance, the first one we teach in the course is the one where the teacher shows her class the two pictures, and tells each class a different one is by Picasso. Based on the info in the stimulus, it could just be the case that the first class preferred picture A and the second class preferred picture B. Despite that possibility (however unlikely), it's the correct answer because there's a lot of support for the correct answer (that some students were affected by the attribution of the picture).