Tukukkk

I want to write "+" and "x" after hline, and I want reducing the space between number and number.

From this

become

This is my code

documentclass[a4paper,12pt,oneside]{letter}

usepackage[left=2.00cm, right=2.00cm, bottom=2.00cm, top=2.00cm]{geometry}

begin{document}

    begin{center}

        begin{tabular}{cccccccc}

            &&&&6&0&0&1\

            &&&&&2&7&2\

            hline$times$

            &&&1&4&0&0&2\

            &&5&2&0&0&7&$cdot$\

            &1&4&0&0&2&$cdot$&$cdot$\

            hline$+$

            &2&1&3&4&2&7&2

        end{tabular}

    end{center}

end{document}

Here is a proposal. Decreasing the distance is standard. In order to place the symbols, I am loading TikZ. Loading TikZ for only this may be overkill.

documentclass[a4paper,12pt,oneside]{letter}

usepackage[left=2.00cm, right=2.00cm, bottom=2.00cm, top=2.00cm]{geometry}

usepackage{tikz}

usetikzlibrary{tikzmark,calc}

begin{document}

    begin{center}

        begin{tabular}{*{8}{@{,}c}}

            &&&&6&0&0&1\

            &&&&&2&7&tikzmarknode{2a}{2}\

            hline %$times$

            &&&1&4&0&0&tikzmarknode{2b}{2}\

            &&5&2&0&0&7&$cdot$\

            &1&4&0&0&2&$cdot$&$tikzmarknode{dot}{cdot}$\

            hline%$+$

            &2&1&3&4&2&7&tikzmarknode{2c}{2}

        end{tabular}

        begin{tikzpicture}[overlay,remember picture]

         node at ([xshift=5mm]$(2a.south)!0.6!(2b.north)$) {$times$};

         node at ([xshift=5mm]$(dot.south)!0.6!(2c.north)$) {$+$};

        end{tikzpicture}

    end{center}

end{document}

To snug up the numerals, all you need to do is add the instruction setlengthtabcolsep{0pt}. If the result looks too tight, maybe try setlengthtabcolsep{0.25pt}.

documentclass[a4paper,12pt,oneside]{letter}

usepackage[margin=2cm]{geometry}

begin{document}

    begin{center}

    setlengthtabcolsep{0pt}  % <-- new

        begin{tabular}{cccccccc}

            &&&&6&0&0&1\

            &&&&&2&7&2\

            hline$times$

            &&&1&4&0&0&2\

            &&5&2&0&0&7&$cdot$\

            &1&4&0&0&2&$cdot$&$cdot$\

            hline$+$

            &2&1&3&4&2&7&2

        end{tabular}

    end{center}

end{document}