Everyone hates rule substitution questions, but I actually think they’re pretty easy, once you practice a bunch and get an intuition for what they’re asking.
The key is to look for the other rules that also affect the rule. Here, there are two other rules that affect M and N:
- Rule 1: M and P can’t go together
- Rule 2: K and N must go together
The answer has to involve two of those four variables. Only answers B, C and D do.
B looks like a good candidate.
C is a lousy candidate. It tells us something that rule 1 already tells us.
D is a lousy candidate. It just uses the same variables from the original rule. You can never replace a rule merely by repeating the same variables.
So only B is left for serious consideration. In fact if you were pressed for time I’d just pick B and be done with it. But let’s examine why B is CORRECT.
The original rule, in it’s full effects, looked like this:
And if you take the contrapositive, you get this:
It changed form slightly, because it’s M(spring) that leads to P(fall). So we can see that if N is in the fall, a result is P in the fall. That’s exactly what B says, so it effectively replaces the original rule.
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GABBY says
Can we please add an explanation on why choice E is incorrect?
Rohan says
But the rule if M: F -> N: S doesn’t allow it to be reversed does it? It specifies the one-way relationship, are we allowed to infer the contra-positive?