\documentclass{article}
\usepackage{calc}
\begin{document}
\section{Counters}
\newcounter{ctra}
\newcounter{ctrb}
\newcounter{ctrc}
\setcounter{ctra}{7/2}
Count 7/2 = \thectra.
\setcounter{ctra}{3*\real{1.6}}
Count 3*1.6 = \thectra.
\setcounter{ctra}{3*\real{1.7}}
Count 3*1.7 = \thectra.
\setcounter{ctra}{3*\real{1.6}*\real{1.7}}
Count 3*1.6*1.7  =\thectra.
\setcounter{ctra}{3+(4*5)}
Count 3+(4*5) = \thectra.
\setcounter{ctra}{3*(4+5)}
Count 3*(4+5) = \thectra.
\section{Lengths}
\newlength{\lena}
\newlength{\lenb}
\newlength{\lenc}
\setlength{\lena}{0pt}
\setlength{\lenb}{300pt}
\setlength{\lenc}{200pt}
\setlength{\lena}{200pt}
Length 200pt = \the\lena.
\setlength{\lena}{2pt+4em}
Length 2pt+4em = \the\lena.
\setlength{\lena}{2em*\real{4.5}}
Length 2em*4.5 = \the\lena.
\setlength{\lena}{2em*\ratio{3cm}{2cm}}
Length 2em*ratio{3cm}{2cm} = \the\lena.
\setlength{\lena}{3pt plus 3pt * \real{1.5}}
Length (3pt plus 3pt)*1.5 = \the\lena.
\setlength{\lena}{\lenc*\ratio{\lenb}{\lenc}}
Length lenc * (lenb/lenc) = \the\lena.
\section{Min \& Max}
\setcounter{ctra}{\minof{2}{4}}
Count min(2,4) = \thectra
\setcounter{ctra}{\maxof{2}{4}}
Count max(2,4) = \thectra
\setcounter{ctra}{3*\maxof{4+5}{3*4}+\minof{2*\real{1.6}}{5-1}}
Count 3*maxof(4+5)(3*4)+minof(2*1.6)(5-1) =\thectra.
\setlength{\lena}{\minof{3pt}{\lenb}*\real{1.5}*\maxof{2*\real{1.6}}{2-1}}
Length minof(3pt)(lenb)*1.5*maxof(2*1.6)(2-1) = \the\lena.
\section{Widths, Heights of Boxes}
\settowidth{\lena}{Some text}
Length of ``Some text'' = \the\lena.
\setlength{\lena}{\widthof{Some text}}
Length of ``Some text'' = \the\lena.
\setlength{\lena}{\widthof{Some text}*\real{0.68}}
Length of ``Some text'' * 0.68 = \the\lena.
\setlength{\lena}{3pt plus 3pt * \real{1.5}}
Length (3pt plus 3pt)*1.5 = = \the\lena.
\section{Macros?}
\def\foo{Other text}
\setlength{\lena}{\widthof{\foo}}
Length of ``\foo = \the\lena.
\def\four{4}
\setcounter{ctra}{3*\maxof{\four+5}{3*\four}}
Count 3*maxof(\four+5)(3*\four) =\thectra.
\end{document}