Difference between revisions of "2012 AMC 10B Problems/Problem 22"

(=Problem 22)
(=Problem 22)
Line 1: Line 1:
 
==Problem 22=
 
==Problem 22=
Let (<math>a_1</math>, <math>a_2</math>, ... <math>a_10</math>) be a list of the first 10 positive integers such that for each <math>2\le</math> <math>i</math> <math>\le10</math> either <math>a_i + 1</math> or <math>a_i-1</math> or both appear somewhere before <math>ai</math> in the list. How many such lists are there?
+
Let (<math>a_1</math>, <math>a_2</math>, ... <math>a_{10}</math>) be a list of the first 10 positive integers such that for each <math>2\le</math> <math>i</math> <math>\le10</math> either <math>a_i + 1</math> or <math>a_i-1</math> or both appear somewhere before <math>a_i</math> in the list. How many such lists are there?
  
  
 
<math>\textbf{(A)}\ \120\qquad\textbf{(B)}\512\qquad\textbf{(C)}\ \1024\qquad\textbf{(D)}\ 181,440\qquad\textbf{(E)}\ \362,880</math>
 
<math>\textbf{(A)}\ \120\qquad\textbf{(B)}\512\qquad\textbf{(C)}\ \1024\qquad\textbf{(D)}\ 181,440\qquad\textbf{(E)}\ \362,880</math>

Revision as of 01:03, 24 December 2012

=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 $a_i$ 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)