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In the PowerBible, it asks you to diagram and provide any inherent inference(s):

Airlines fly a route if and only if the route is profitable.

(AFR = airline flies route; RP = route profitable)

Diagram: AFR <-------> RP

Inherent Inferences:

AFR --most--> RP
RP --most--> AFR
AFR --some--> RP

My question is why isn't "RP --some-->AFR" a valid inherent inference?

Thank you for your enlightenment.

It is valid. Since it's equivalent to AFR --some--> RP, the author didn't need to restate it. Relationships including the term "some" go both ways. For instance, if we know that some As are Bs, we also know that some Bs are As.

Cambridge has it right. I had the same problem as well when I first started. Once the some diagrammed "train" is established, the connections between the two variables can go either way without an issue.