Seven people enter a lift. The lift stops at only three (unspecified) floors. At each of the three floors no one enters, but at least one person leaves. After the three stops the lift is empty. In how many ways can this happen?
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Correct answer: C. 1806
Explanation
Count onto functions from 7 people to 3 floors: 3⁷ − 3·2⁷ + 3·1⁷ = 2187 − 384 + 3 = 1806.More Discrete Structures and Optimisation questions
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