\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{enumitem}
\usepackage{tikz}
\pgfmathsetseed{\number\pdfrandomseed}
\usepackage{xlop}
\usepackage[user]{zref}
\usepackage{fancyhdr}

\makeatletter
%%=============================================================================
%%  \DefineVariableWithEachAndDo{<macro holding list of undelimited arguments>}%
%%                              {<macro holding current element of list>}%
%%                              {<tokens>}
%%=============================================================================
\newcommand\UD@Exchange[2]{#2#1}%
\newcommand\UD@PassFirstToSecond[2]{#2{#1}}%
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\@ifdefinable\UD@stopromannumeral{\chardef\UD@stopromannumeral=`\^^00}%
\newcommand\DefineVariableWithEachAndDo[3]{%
  \romannumeral
  \expandafter\UD@Exchange
  \expandafter{#1}{\DefineVariableWithEachAndDoloop{#2}{#3}{}}{\relax}%
}%
\newcommand\DefineVariableWithEachAndDoloop[4]{%
  \ifx\relax#4\expandafter\UD@firstoftwo\else\expandafter\UD@secondoftwo\fi
  {\UD@stopromannumeral#3}{%
    \DefineVariableWithEachAndDoloop{#1}{#2}{#3\def#1{#4}#2}%
  }%
}%
%%=============================================================================
\zref@newprop{AddOnea}{0}%
\zref@newprop{AddTwoa}{0}%
\zref@newprop{range}{0}%
\newcommand\undefinedstyle[1]{\nfss@text{\reset@font\bfseries ??}}%
\newcommand\undefinedcarrystyle[1]{{\nfss@text{\reset@font\scriptsize\bfseries ??}}}%
\newcommand\phantomcarrystyle[1]{\phantom{\scriptsize{#1}}}%
\newcommand{\AddQuestions}[2]{%
    \zref@setcurrent{range}{#1}%
    \zref@labelbyprops{#2}{range}%
    \foreach \i in {1,...,#1}{%
         \par\ifnum\i=1 \else\medskip\fi\noindent\hbox{Question \i: }%
         \zref@ifrefundefined{#2.\i}{%
           \pgfmathtruncatemacro{\AddOnea}{random(\DifficultyOne)}%
           \pgfmathtruncatemacro{\AddTwoa}{random(\DifficultyTwo)}%
           \expandafter\UD@PassFirstToSecond\expandafter{\AddOnea}{\zref@setcurrent{AddOnea}}%
           \expandafter\UD@PassFirstToSecond\expandafter{\AddTwoa}{\zref@setcurrent{AddTwoa}}%
         }{]
           \expandafter\expandafter\expandafter\UD@PassFirstToSecond
           \expandafter\expandafter\expandafter{%
             \zref@extractdefault{#2.\i}{AddOnea}{0}%
           }{\zref@setcurrent{AddOnea}}%
           \expandafter\expandafter\expandafter\UD@PassFirstToSecond
           \expandafter\expandafter\expandafter{%
             \zref@extractdefault{#2.\i}{AddTwoa}{0}%
           }{\zref@setcurrent{AddTwoa}}%
         }%
         \zref@labelbyprops{#2.\i}{AddOnea, AddTwoa}%
         \zref@refused{#2.\i}%
         \zref@ifrefundefined{#2.\i}{%
           % actually setting carrystyle is not needed as there are no carries with defaults 0, but ...
           \opadd[carryadd=true, voperator=bottom, resultstyle=\undefinedstyle, operandstyle=\undefinedstyle, carrystyle=\undefinedcarrystyle]%
         }{%
           % Only operands shall be seen, so turn everything else into \phantom
           \opadd[carryadd=true, voperator=bottom, resultstyle=\phantom, carrystyle=\phantomcarrystyle]%
         }%
         {\zref@extractdefault{#2.\i}{AddOnea}{0}}%
         {\zref@extractdefault{#2.\i}{AddTwoa}{0}}%
    }%
    \par
}%
\newcommand{\AddAnswers}[1]{%
    \zref@ifrefundefined{#1}{%
      \zref@refused{#1}%
      \par\noindent
      {\normalfont\bfseries [Question-series ``#1'' undefined, probably re-run latex, see messages on console/in .log-file]}%
      \par
     }{%
      \expandafter\UD@PassFirstToSecond\expandafter{%
         \romannumeral
         \expandafter\expandafter\expandafter\UD@Exchange
         \expandafter\expandafter\expandafter{%
            \zref@extractdefault{#1}{range}{1}%
         }{\UD@stopromannumeral 1,...,}%
      }{\foreach \i in }{%
           \par\ifnum\i=1 \else\medskip\fi\noindent\hbox{\rlap{Answer \i: }\phantom{Question \i: }}%
           \zref@refused{#1.\i}%
           \opadd[carryadd=true, voperator=bottom]%
                 {\zref@extractdefault{#1.\i}{AddOnea}{0}}%
                 {\zref@extractdefault{#1.\i}{AddTwoa}{0}}%
      }%
    }%
    \par
}%
\newcommand*{\DifficultyOne}{999}%
\newcommand*{\DifficultyTwo}{99}%
%%=============================================================================
%% A dirty hack for resetting page-numbers with each sheet
\newcommand\resetpagezreflabelnamenumber{0}%
\newcommand\resetpage{%
  \newpage
  \xdef\resetpagezreflabelnamenumber{\number\numexpr\resetpagezreflabelnamenumber+1\relax}%
  \zref@labelbyprops{pagereset-\resetpagezreflabelnamenumber}{page}%
}%
\newcommand\obtainlastresetpage{%
  \zref@extractdefault{pagereset-\resetpagezreflabelnamenumber}{page}{0}%
}%
\newcommand\ps@resetableplain{%
  \let\@mkboth\@gobbletwo
  \let\@oddhead\@empty
  \def\@oddfoot{\reset@font\hfil\@arabic{\number\numexpr\value{page}-\obtainlastresetpage+1\relax}\hfil}%
  \let\@evenhead\@empty
  \let\@evenfoot\@oddfoot
}%

\makeatother

\pagestyle{resetableplain}

\newcommand\ExamineeNameList{%
  {Coby}%
  {Johan}%
  {Rudy}%
  {Rodrigo}%
  {Alejandra}%
  {Oswaldo}%
  {Yusuf}%
  {Luz}%
  {Denzel}%
  {Abigayle}%
  {Bobby}%
  {Margaret}%
  {Eli}%
  {Billy}%
  {Jazmine}%
  {Miguel}%
  {Jon}%
  {Valentin}%
  {Lance}%
  {Skyler}%
}%
\newcommand\ThisExamimeesName{}% <- This raises an error if the macro is already in use by some package/by some other code.

\begin{document}

%%%%%%%
%% Create a sheet of 15 ramdom questions for each examinee:
\DefineVariableWithEachAndDo{\ExamineeNameList}{\ThisExamimeesName}{%
  \resetpage
  \section*{Question sheet without answers for \ThisExamimeesName}%
  \AddQuestions{15}{This question-series-sheet-\ThisExamimeesName-label}%
}%
%%%%%%%
%% Re-create the sheets from above, this time with answers:
\DefineVariableWithEachAndDo{\ExamineeNameList}{\ThisExamimeesName}{%
  \resetpage
  \section*{Question sheet with answers for \ThisExamimeesName}%
  \AddAnswers{This question-series-sheet-\ThisExamimeesName-label}%
}%

\end{document}