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2021 AMC 10A Problems/Problem 20

Revision as of 15:08, 11 February 2021 by Contactbibliophile (talk | contribs)

Problem

In how many ways can the sequence $1,2,3,4,5$ be rearranged so that no three consecutive terms are increasing and no three consecutive terms are decreasing? $\textbf{(A)} ~10\qquad\textbf{(B)} ~18\qquad\textbf{(C)} ~24 \qquad\textbf{(D)} ~32 \qquad\textbf{(E)} ~44$

Solution (bashing)

We write out the 120 cases. [hide=The Cases] $1,2,3,4,5$ $1,2,3,5,4$ $1,2,4,3,5$ $1,2,4,5,3$ $1,2,5,3,4$ $1,2,5,4,3$ $1,3,2,4,5$ $1,3,2,5,4$ $1,3,4,2,5$ $1,3,4,5,2$ $1,3,5,2,4$ $1,3,5,4,2$ $1,4,2,3,5$ $1,4,2,5,3$ $1,4,3,2,5$ $1,4,3,5,2$ $1,4,5,2,3$ $1,4,5,3,2$ $1,5,2,3,4$ $1,5,2,4,3$ $1,5,3,2,4$ $1,5,3,4,2$ $1,5,4,2,3$ $1,5,4,3,2$ $2,1,3,4,5$ $2,1,3,5,4$ $2,1,4,3,5$ $2,1,4,5,3$ $2,1,5,3,4$ $2,1,5,4,3$ $2,3,1,4,5$ $2,3,1,5,4$ $2,3,4,1,5$ $2,3,4,5,1$ $2,3,5,1,4$ $2,3,5,4,1$ $2,4,1,3,5$ $2,4,1,5,3$ $2,4,3,1,5$ $2,4,3,5,1$ $2,4,5,1,3$ $2,4,5,3,1$ $2,5,1,3,4$ $2,5,1,4,3$ $2,5,3,1,4$ $2,5,3,4,1$ $2,5,4,1,3$ $2,5,4,3,1$ $3,1,2,4,5$ $3,1,2,5,4$ $3,1,4,2,5$ $3,1,4,5,2$ $3,1,5,2,4$ $3,1,5,4,2$ $3,2,1,4,5$ $3,2,1,5,4$ $3,2,4,1,5$ $3,2,4,5,1$ $3,2,5,1,4$ $3,2,5,4,1$ $3,4,1,2,5$ $3,4,1,5,2$ $3,4,2,1,5$ $3,4,2,5,1$ $3,4,5,1,2$ $3,4,5,2,1$ $3,5,1,2,4$ $3,5,1,4,2$ $3,5,2,1,4$ $3,5,2,4,1$ $3,5,4,1,2$ $3,5,4,2,1$ $4,1,2,3,5$ $4,1,2,5,3$ $4,1,3,2,5$ $4,1,3,5,2$ $4,1,5,2,3$ $4,1,5,3,2$ $4,2,1,3,5$ $4,2,1,5,3$ $4,2,3,1,5$ $4,2,3,5,1$ $4,2,5,1,3$ $4,2,5,3,1$ $4,3,1,2,5$ $4,3,1,5,2$ $4,3,2,1,5$ $4,3,2,5,1$ $4,3,5,1,2$ $4,3,5,2,1$ $4,5,1,2,3$ $4,5,1,3,2$ $4,5,2,1,3$ $4,5,2,3,1$ $4,5,3,1,2$ $4,5,3,2,1$ $5,1,2,3,4$ $5,1,2,4,3$ $5,1,3,2,4$ $5,1,3,4,2$ $5,1,4,2,3$ $5,1,4,3,2$ $5,2,1,3,4$ $5,2,1,4,3$ $5,2,3,1,4$ $5,2,3,4,1$ $5,2,4,1,3$ $5,2,4,3,1$ $5,3,1,2,4$ $5,3,1,4,2$ $5,3,2,1,4$ $5,3,2,4,1$ $5,3,4,1,2$ $5,3,4,2,1$ $5,4,1,2,3$ $5,4,1,3,2$ $5,4,2,1,3$ $5,4,2,3,1$ $5,4,3,1,2$ $5,4,3,2,1$ [/hide] Some quick work gets us $\boxed{\text{D: }32}$ permutations that work. ~contactbibliophile

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