Formatting a comma separated list of values

Question

I'm creating a set of mathematics worksheets, and I'd like to have an upside down "answer key" below a set of enumerated questions. Here is what I have at the moment:

\usepackage{graphicx}

Answers:

\rotatebox{180}{$1. \underline{5}$ $2.\underline{5}$
$3.\underline{10}$ $4.\underline{8}$}

This produces the following:

Given that all the answers are in the same format (problem number, then an underlined answer), is there a way to create a command that would produce the formatted text (I can add the \rotatebox myself) for a set of values?

I'm thinking something like this:

\createanswerbox[5,5,10,4]

Answer 1

Taking some hints from Package xparse \SplitList last token, you can define a list processor that performs your request iteratively:

\documentclass{article}
\usepackage{graphicx,xparse}% http://ctan.org/pkg/{graphicx,xparse}
\newcounter{itemcntr}
\NewDocumentCommand\createanswerbox{O{,\,} >{\SplitList{,}}m}
{%
  \setcounter{itemcntr}{0}% Start at 1.
  \def\itemdelim{\def\itemdelim{#1}}% Define list separator with one delay
  \ProcessList{#2}{\myitem}% Process list
}
\newcommand\myitem[1]{\stepcounter{itemcntr}\itemdelim\theitemcntr.\underline{#1}}

\begin{document}
\createanswerbox{5,5,10,4}

\rotatebox{180}{\createanswerbox{5,5,10,4}}
\end{document}