\documentclass[12pt]{article}
\usepackage{amsmath}
%\usepackage{fullpage}
\usepackage[top=1in, bottom=1in, left=0.8in, right=1in]{geometry}
\usepackage{multicol}
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\setlength{\columnsep}{0.1pc}

\title{cs109 Problem Set}
\author{Your Name -- \texttt{sunetid@stanford.edu} -- STUDENTID}
\date{\today}
\begin{document}

  \maketitle

  \vspace{-0.3in}
  \rule{\linewidth}{0.4pt}

  \textbf{For each problem, briefly explain/justify how you
  obtained your answer.} This will help us determine your understanding of
  the problem whether or not you got the correct answer. Moreover, in the
  event of an incorrect answer, we can still try to give you partial credit
  based on the explanation you provide. It is fine for your answers to include
  factorials, exponentials, or combinations; you don't need to calculate those
  all out to get a single numeric answer.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Problems:

  \begin{enumerate}

    % Problem 1
    \item Problem 1 desctiption

    \begin{enumerate}
      \item Problem 1a description

% Your answer for 1.a


      \item Problem 1b description

% Your answer for 1.b


      \item Problem 1c description

% Your answer for 1.c

    \end{enumerate}


    % Problem 2
    \item Problem 2 description

% Your answer for Problem 2


    % Problem 3
    \item Problem 3 description


% The examples above are meant to show you the structure of the document,
% where you would be answering multiple questions, including potentially
% questions that have sub-parts.


  \end{enumerate}

\end{document}