Q17

[mhallett06]

I am having trouble with number 17 in this game. I believe both A and E must be false. I am sure the problem lies within the inferences I created. Please help. I set up the following rules and inferences:

Rules:
M->L or S
L->G
L<+>P
S->P
S<+>Z
G->H and Z

Inferences:
L<->G
L<->H (b/c if L then G. If G then H and Z)
L<->Z
S<+>L (b/c if L not P. If S then P)
S<+>Z (combine 2nd and 6th rules with 4th inference.)
S<+>G (same as above)

if, then: ->
if and only if: <->
not together: <+>

[ManhattanPrepLSAT1]

Here's a couple of problems with the rules that you've set up as well as the inferences you drew.

The first constraint requires that literature and sociology cannot both be offered when mathematics is offered. Your setup doesn't reflect that.

Also, many of your inferences commit "reversals" of conditional statements. Remember that A --> B does not imply B --> A.

I've posted a setup for #17 to help you see it. Good luck!

My advice... maybe just try not drawing inferences from the conditional constraints, setup the logic chain, and just follow along.

[tzyc]

How can we find out which hypothesis to test? (In this case to test if G is offered)
Or we just need to test each answer choice?

Thanks

[timmydoeslsat]

You can approach this kind of question in two ways.

1) Use previous work from local questions (if questions) to accumulate evidence to disprove answer choices.

2) Realize that it is a global question and that there is a great chance it could be answered with a strong reading of the global setup/diagram.

Here is how I a diagrammed this binary grouping game:



We want to notice logical contradictions.

For instance, if I told you that this is true in the world:

A ---> ~C

X ---> C

We would know that A and X could not exist together, as they lead to a logical contradiction. You cannot have both ~C and C occur.

So, when we look at the diagram of this game above, we notice that L and S could never occur together, as they, together, lead to P and ~P. We can also see that G and S lead to a logical contradiction: Z and ~Z. So we know that G and S cannot be in together.

So it must be false that G and S are in.

[tzyc]

Thanks for your great response, timmydoeslsat.
A quick question-did you find the logic (both X and A cannot be offered because then there would be contradiction) and how to get to the correct answer by looking through the answer choices?
Because all answer choices say X and Y cannot be both offered in this case...so, I felt for "global question" maybe we want to look through answer choises and find the commonality, and then what logic could be used, and based on the logic, find out which answer is correct/wrong- is this understanding correct?
And a global question is a broad question, covers broad range of elements, right?

[timmydoeslsat]

I would have seen that this was a must be false question stem and that the answer choices were centered around the idea of two things being together. It is then the most prudent thing to come up with the answer yourself in my opinion. Based on the limited rules, you can quickly infer that the answer will either be L and S...or G and S.

Now this must be false question could have been centered around the idea of something that "G is in with 3 courses." This is not something that is going to test the concept described above. But since this question is going after an unacceptable two course selection, we can go to the rules and look for logical contradictions.