\documentclass[a4paper]{article}

% -----------------------
% Macros for time reports

% Use the fixed point arithmetic
% package by Michael Mehlich.
\usepackage{fp}

% Initialize counter.
\FPset{\hours}{0}

\newcommand{\addhour}[1]{
    % Increase counter by given value.
    \FPadd{\hours}{\hours}{#1}
}

\newcommand{\duration}[1]{
    % Increase counter.
    \addhour{#1}
    % Show value it is increased by.
    #1 hours
}

\newcommand{\totalduration}{
    % Round output to one digit after the dot.
    \FPround{\hours}{\hours}{1}
    % Show total value.
    \FPprint{\hours} hours
}

% -------------------
% An example document

% Suppress indention of paragraphs.
\setlength{\parindent}{0em}

\begin{document}
    \duration{3} \\
    \duration{4.5} \\
    \duration{7} \\
    \\
    \textbf{\totalduration}
\end{document}