2012 AMC 10B Problems/Problem 22

Revision as of 00:03, 24 December 2012 by B1A0 (talk | contribs) (=Problem 22)

=Problem 22

Let ($a_1$, $a_2$, ... $a_10$) be a list of the first 10 positive integers such that for each $2\le$ $i$ $\le10$ either $a_i + 1$ or $a_i-1$ or both appear somewhere before $ai$ in the list. How many such lists are there?


$\textbf{(A)}\ \120\qquad\textbf{(B)}\512\qquad\textbf{(C)}\ \1024\qquad\textbf{(D)}\ 181,440\qquad\textbf{(E)}\ \362,880$ (Error compiling LaTeX. Unknown error_msg)