if a and b, then c?? how to diagram

[WeixinY906]

this rule appeared in two games in Dec Asia test!! I wasn't able to fit it into the in/out diagram and this might directly kill my could-have-been useful Dec. score...

so basically it was about a kid choosing courses to take in either semester 1 or 2. there were two rules like
if m and g in Semester 1, then l in Semester 2

the correct contrapositive should be
if l not in S2(i.e. in S1)--> m in S2/g in S2/ m.g both in S2

I did not realize this until the last question of the game and assumed m.g. both in S2 all the way...how dead do you guys think I am?

Also, in general how to deal with & diagram rules like this?

[ohthatpatrick]

Hopefully messing up that rule cost you at most 3 questions, which means at most it cost you 1 or 2 points on your actual score.

When you have a conditional and you're deriving the contrapositive, you always switch "and" into "or" and vice versa.

So given
A and B --> C
you get
~C -> ~A or ~B

You want to think of 'negating' the words 'and' and 'or', just like you're negating the other ideas.

Given
H --> ~K or M
you get
K and ~M --> ~H

If you're using the Manhattan Prep logic chain, it would be hard to incorporate a rule that has 'and' on the left side.

You can just write that conditional off to the side, or you can just eschew using the logic chain and just write all your rules as conditionals off to the side.

Hope this helps.

[WeixinY906]

Hopefully messing up that rule cost you at most 3 questions, which means at most it cost you 1 or 2 points on your actual score.

When you have a conditional and you're deriving the contrapositive, you always switch "and" into "or" and vice versa.

So given
A and B --> C
you get
~C -> ~A or ~B

You want to think of 'negating' the words 'and' and 'or', just like you're negating the other ideas.

Given
H --> ~K or M
you get
K and ~M --> ~H

If you're using the Manhattan Prep logic chain, it would be hard to incorporate a rule that has 'and' on the left side.

You can just write that conditional off to the side, or you can just eschew using the logic chain and just write all your rules as conditionals off to the side.

Hope this helps.

Thank you!
But what about the ~C--> ~A & ~B situation? isn't it one possibility too?

[ohthatpatrick]

I'm not sure I know what you're asking.

I think you're saying, "When C is out, couldn't both A and B be out?"

Yes, definitely. When you write/read "or" on LSAT, you always interpret it to mean "AT LEAST one of these is true".
"Or" is only setting up a mutually exclusive situation if you specify "but not both".

So when you write the rule
~C -> ~A or ~B
you'd interpret that as
"When C is out, AT LEAST one of A and B is out."