Cannot be true/cannot do specific actions questions

[tzyc]

Hello,
I found I'm really not good at those questions...
Such as X cannot have which one of the two or which cannot be the third etc...
What or where should I pay attention to?
I tried to find contradictions between constraints, and I can get easy ones correct but especially when timed, I cannot see contradictions immediately many times and become upset...
Could anyone give me advice?
Thank you.

[timmydoeslsat]

Please give me a specific game that you recently did where this was an issue.

[ManhattanPrepLSAT1]

If you have a hard time seeing why certain answer choices would violate the constraints, don't get frustrated, that's actually true for me too. Play out the answer choices one at a time and see what happens.

Go to answer choice (A), and check to see if it could be true by creating a hypothetical that satisfies the constraints. If it works, move to answer choice (B), and so on. The issue is trial and error, while not so sophisticated, is the fastest way to go sometimes - especially if you can't see implications of the answer choice.

[tzyc]

>>Timmy
I asked specific ones on each category before and you already answered them, thanks.
Let me post ones when I meet some later...I posted this post because I wanted to know whether there is a general rule to find contradictions between constraints or how to find a key to see what cannot be true because I missed this kind of questions a lot...

>>Matt
Thank you for your reply. I'm not sure whether I have time for trying each answer choice but I will keep in mind to use the way if I cannot see the contradiction.

[timmydoeslsat]

In a broad sense, global questions asking what cannot be true center around the idea of logical contradictions.

For instance, say you had a binary grouping game with 5 variables: ABCDE

The rules are:

B ---> C

E ---> ~D

A ---> ~C

We can see that having both A and B in the "in" column/group would lead to a logical contradicition in which C would both be in and out. Since such a thing is not possible, we must have at least one of A and B out.

[tzyc]

I know we must find a contradiction but was not sure which constraint to focus.
But ah that's a good example...and I assume we should be careful with X→Y constraint, since the one you helped me before also involved this type of constraint.

Thanks!