Unit 12 · Multi-Step Conditional Reasoning
Robbie & Declan — Who Crosses the Korra River?
Lesson 16 of 16
About this lesson
Three ways to cross the Korra River — two bridges on alternating days, and a ferry that always runs on Sundays but can also substitute when a bridge is damaged. Robbie thinks the ferry running two days in a row proves both bridges are broken. Declan thinks Archer Bridge is the only option on Fridays. Both are wrong. This question rewards students who think in counter-examples: "Can I find even one situation where their claim fails?"
Chapters
Question
| There are three ways to cross the Korra River:
Archer Bridge is only open on Mondays, Wednesdays and Fridays.
Djorlom Bridge is only open on Tuesdays, Thursdays and Saturdays.
| The Korra Ferry is used on Sundays. It is also sometimes used on other days, but only if the bridge that would normally be open is damaged. |
|---|
Robbie: "If the ferry operates for two days in a row, it must mean that both the bridges are damaged."
Declan: "If you don't use the Archer Bridge, you'll never be able to cross the river on Friday."
If the information in the box is true, whose reasoning is correct?
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