2022 AMC 10B Problems/Problem 23
Solution
We use the following lemma to solve this problem.
Let 
be independent random variables that are uniformly distributed on 
. Then for 
,
![\[ \Bbb P \left( y_1 + y_2 \leq 1 \right) = \frac{1}{2} . \]](http://latex.artofproblemsolving.com/9/c/a/9ca0d2902c5eb8db92cddab19e3ca4e92b982504.png)
For 
,
![\[ \Bbb P \left( y_1 + y_2 + y_3 \leq 1 \right) = \frac{1}{6} . \]](http://latex.artofproblemsolving.com/f/7/e/f7e0ecac9af7c72deadc3b60deae91f69748cb18.png)
Now, we solve this problem.
We denote by 
the last step Amelia moves. Thus, 
.
We have
$$ (Error compiling LaTeX. Unknown error_msg)
\begin{align*}
\Bbb P \left( \sum_{n=1}^\tau x_n > 1 \right)
& = \Bbb P \left( x_1 + x_2 > 1 \bigg| t_1 + t_2 > 1 \right)
\Bbb P \left( t_1 + t_2 > 1 \right) \\
& \hspace{1cm} + \Bbb P \left( x_1 + x_2 + x_3 > 1 \bigg| t_1 + t_2 \leq 1 \right)
\Bbb P \left( t_1 + t_2 \leq 1 \right) \\
& = \Bbb P \left( x_1 + x_2 > 1 \right)
\Bbb P \left( t_1 + t_2 > 1 \right)
+ \Bbb P \left( x_1 + x_2 + x_3 > 1 \right)
\Bbb P \left( t_1 + t_2 \leq 1 \right) \\
& = \left( 1 - \frac{1}{2} \right)\left( 1 - \frac{1}{2} \right)
+ \left( 1 - \frac{1}{6} \right) \frac{1}{2} \\
& = \boxed{\textbf{(C) 
}} .
\end{align*}
$$ (Error compiling LaTeX. Unknown error_msg)
where the second equation follows from the property that 
and 
are independent sequences, the third equality follows from the lemma above.
~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)
Video Solution
~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)
