Q23

[ManhattanPrepLSAT1]

[moshemeer]

why is E not the same as C?

[ohthatpatrick]

(E) is just an illegal reversal of the rule we're replacing.

They asked us to replace
~K --> H and M

and (E) says
H and M --> ~K

Are you familiar enough with conditional logic to know what I mean by an illegal reversal?

The same answer applies to why (E) and (C) are different.

(C) starts with ~K

(E) ends with ~K

[priyanka.krishnamurthy]

Quick question on E... I use the 'if not' = 'unless' method when diagramming unless. I just want to confirm that 'if not either H or M' would translate to 'if neither H and M' That's the only way I am getting that mistaken reversal & I just want to make sure I got that straight going forward.

Diagram:
~H and ~M -> T

Thanks!
pK

[ohthatpatrick]

Sorry for the delayed answer to the quick question:

YES, when we say “if not” before a compound idea, such as “H or M”, you’re saying “If that compound idea is not the case”, which is essentially the same as doing what you’d normally do when you negate ‘H or M’ to write a contrapositive.

IF NOT “H or M” = ~H and ~M
IF NOT “G and ~F” = ~G or F

In fact, the method some people use for “unless” is to just write the two ideas in the sentence as the two right side ideas.

Given:
Unless either H or M goes to T, K goes to T

We could write:
---> Ht or Mt
---> Kt

And then just contrapose those to see the left sides:
…………….~Kt --> Ht or Mt
~Ht and ~Mt --> Kt


More conversationally, you can think, “What has to be true for it to NOT BE THE CASE that either H or M is assigned to T? I guess they would BOTH have to be NOT ASSIGNED to T.”

[YuriJ257]

I don't get the conditional logic behind AC C. Please help.

I thought it was both F and M must be T unless K is T which translates to:
if F & M are ~T, K is T.
if K is ~ T, F or M are T.

SO, if F OR M can be T if K is NOT T, then it could be:

S T
K M
F G
H
L

No...? Is this analysis incorrect? Please help explain C. I just don't get it.

[ohthatpatrick]

You said:
I thought it was both F and M must be T unless K is T which translates to:
if F & M are ~T, K is T.

That's incorrect. "unless" always means "put the negated version of this idea on the left side of the arrow",

Unless X happens, Y happens
~X --> Y

G happens, unless H doesn't happen
H ---> G

M happens, unless P and K happen
~ (P and K) --> M
which means ...
~P or ~K ---> M

Because of that,
(C) would be written like this:
~Kt --> Ft and Mt

is that the same as the original rule?
~Kt ---> Ht and Mt

It's pretty close, other than switching Ft and Ht.
But wait! Since there is a rule that F and H are always in the same group, those mean the same thing!

Thus, it is the same rule.

Incidentally, this is the biggest pattern so far for correct answers to Equivalent Rule questions: whichever rule they're asking us to replace contains some character (A) who is in SOME OTHER rule, inextricably bound up with some other character (B).

The correct answer re-writes the original rule by swapping out A for B.

For example, if you had these two rules:
F - J
and
GJ

Then if they asked us to replace "F - J", the correct answer would likely be "F - G".

Hope this helps.

[AnaC992]

How would you solve this problem using the rule-breaking method (break the original rule and test out the answer choices)? I had trouble eliminating C when I first did this question using this method. Thank you!