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Problem 211

Combinatorics Difficulty 1.4 Multiple choice CEMC Fermat · Canada · 2024

In a photograph, Aristotle, David, Flora, Munirah, and Pedro are
seated in a random order in a row of $5$ chairs. If David is seated in the
middle of the row, what is the probability that Pedro is seated beside
him?

Pick one

Official solution

After David is seated, there are $4$ seats in which Pedro can be seated, of
which $2$ are next to David.

Thus, the probability that Pedro is next to David is $\frac{2}{4}$ or $\frac{1}{2}$ .

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.