Discrete Mathematics and Its Applications, Seventh Edition

Published by McGraw-Hill Education
ISBN 10: 0073383090
ISBN 13: 978-0-07338-309-5

Chapter 1 - Section 1.2 - Applications of Propositional Logic - Exercises - Page 23: 17

Answer

The hostess knows to bring two coffees because both professors 1 and 2 consider the option where everyone wants coffee to be viable, but professor 3 knows there is no way for everyone to want coffee; hence, she does not want it.

Work Step by Step

Professors 1, 2, 3 answers: IDK, IDK, and No, not everyone wants coffee. The hostess brings back two coffees. If professor 1 or 2 did not want coffee, they would be able to answer the question "Does everyone want coffee?" with a firm "No." By this logic, the third professor clearly does not want coffee.
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