Unless with 2 sufficient terms

[lsatquestions]

Example from Kaplan textbook:
"Unless it rains, we will go either to the beach OR the park"

Unless signals the necessary term = rain
The book translates the statement into If NOT beach AND NOT park --> rain.

Does the use of unless both negate the sufficient term and switch OR into AND and vice versa (in just the translation, not the contrapositive)? Doesn't say this anywhere in the textbook.

[Jon Denning]

Welcome to the Forum and thanks for posting!

You ask a great question, and one that I'm disappointed to hear that Kaplan book doesn't do a better job of explaining (as a point of comparison, we spend a fair amount of time in the Logical Reasoning Bible outlining "unless"-specific strategies for situations precisely like the one you describe, so it's a shame others don't delve into it as comprehensively, if at all).

There are ultimately two ways to approach a phrase like, "Unless it rains, we will go either to the beach or the park." Let's look at both!

1. Use "unless" as the necessary condition indicator and then negate everything else, including the remaining conditions and any linking term like "or" or "and." So "rain" becomes necessary, then "go to the beach" becomes "NOT go to the beach," "go to the park" becomes "NOT go to the park," and the linking term "or" becomes its negated form, "and." Those pieces then all jump to the sufficient side, preceding our arrow:

Go to beach AND Go to park Rain

2. Think of "unless" as an equivalent form, "If not..." With this approach you get the sufficient condition statement, "If NOT rain..." which goes in front of our arrow, and then the rest becomes necessary exactly as given: "Go to the beach OR go to the park":

Rain Go to beach OR Go to park


You'll notice if you look at the two diagrams above that they're contrapositives of one another, which is exactly what you should get if the statement in question has been diagrammed correctly: the two forms of it should be identical in meaning, i.e. contrapositives

So, sorry again that Kaplan was so light on details, but hopefully this explanation helps straighten it all out!