June 1991 LSAT Section 3 Question 19

Train service suffers when a railroad combines commuter and freight service. By dividing its attention between its fr...

Harper on August 8, 2020

Help please

I am confused with answer choice C. According to the lesson on sufficient and necessary, the part of the of the sentence that follows unless becomes the necessary and the other part gets negated and is the sufficient. Obviously here both parts are after “unless”, so what would be a better rule to learn this. A tutor taught me that the necessary is whatever is modified by “unless” but I don’t think that helps much here either.

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shunhe on August 11, 2020

Hi @Harper,

Thanks for the question! So “what comes after unless” is still good advice, but you have to think of it as “what comes directly after unless.” So your tutor probably told you

Y unless X

can be diagrammed

~X —> Y
or
~Y —> X

but when we have “unless X, Y”, it’s the same thing. Why? Because we can just rewrite it

Y Unless X.

For example, here, take a look at (C). Unless a railroad serves its customers well, it will not be a successful business. Well, that’s just basically saying that a railroad won’t be a successful business unless it serves its customers well! We can just reword that, and then we can diagram it with the usual rule.

If it helps, you can also think of “unless” as “if not.” So then it’d be

“If a railroad doesn’t serve its customers well, it won’t be a successful business”

And then you’d just diagram that the same way as any “if X, then Y” statement.

Hope this helps! Feel free to ask any other questions that you might have.