Unit 8 · Detecting Reasoning Errors
River Crossing — Three Ways Logic
Lesson 31 of 34
About this lesson
Three ways to cross the Korra River, each with its own schedule and rules. Amanda thinks three consecutive ferry days means both bridges are broken. Declan thinks he can only ever cross on Sundays. Both are making absolute claims — and both can be shown wrong with one counterexample.
Chapters
Question
There are three ways to cross the Korra River:
Archer Bridge is open to cars and pedestrians. It is only open on Mondays, Wednesdays and Saturdays. It is one of the highest bridges in Australia.
Djorlom Bridge is open to cars, but not pedestrians. It is only open on Tuesdays, Thursdays and Fridays.
The Korra Ferry is for pedestrians only. It is open on Sundays. It is also sometimes open on other days, but only if the bridge that would normally be open is damaged.
Amanda said: “If the ferry operates for three days in a row, it must mean that both bridges are damaged.”
Declan said: “I don’t have a car, and I’m terrified of heights. Unless I get someone to drive me, I’ll only ever be able to cross on a Sunday.”
If the information in the box is true, whose reasoning is correct?
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