newcounter{num}
newcommand{soal}[2]{
    stepcounter{num} thenum & {#1} & {#2} \[4pt] hline
}

documentclass{article}
usepackage{calculator}
usepackage{ifthen}
newboolean{firstTime}                      % Declare a new variable of type boolean
setboolean{firstTime}{true}                % Set it to true
newcounter{num}

newcommand{soal}[2]{
    ifthenelse{boolean{firstTime}}{       % If first time, add 0 + (passed number)
        ADD{0}{#2}{totalMarks}            % Perform the actual calculation
        setboolean{firstTime}{false}       % Set the boolean variable to false
    }{                                      % Else, a total exists, so add passed number
        ADD{totalMarks}{#2}{totalMarks}  % Perform the actual calculation
    }
    begin{tabular}{ccc}
        stepcounter{num} thenum & {#1} & totalMarks \[4pt] hline
    end{tabular}par
}
begin{document}
    soal{Started at two and $frac{1}{2}$}{2.50}
    soal{Added three and $frac{1}{4}$}{3.25}
    soal{Added five and $frac{3}{4}$}{5.75}par~\[2ex]
    Now, for the total marks: totalMarks
end{document}

documentclass{article}
newcounter{num}
newcounter{total}

newcommand{soal}[2]{
    begin{tabular}{ccc}
        stepcounter{num} thenum & {#1} & addtocounter{total}{#2} thetotal \[4pt] hline
    end{tabular}par
}
begin{document}
    soal{Started at two}{2}
    soal{Added three}{3}
    soal{Added five}{5}
end{document}