How can i extend header row and summary lines of table?

I want to extend header row and summary lines of table to page width. Please someone help me with this, i don’t know how to do it.

Here is my existing table:

The table i want:

My Code:

begin{longtable}{l@{extracolsep{fill}hspace{tabcolsep}} p{.6textwidth} l r r r r }
    hline
    rowcolor{black}
    textcolor{white} {POS} &textcolor{white} {DESCRIPTION} & textcolor{white} {QTY} & textcolor{white} {PRICE (pounds)} & textcolor{white} {AMOUNT  (pounds)} *
    hlinehline
    endhead
}{

    & & & {MWSt taxval %} & {taxes{arabic{cost}}{taxval}} 
    hlinehlinehline
    & & &  {Summe EUR} & {addtaxes{arabic{cost}}{taxval}} 
    end{longtable}

documentclass[a4paper,10pt]{article}

usepackage{geometry}
begin{document}
marginparwidth 0pt
marginparsep 0pt
oddsidemargin dimexpr .5in -1in
textwidth dimexpr pdfpagewidth oddsidemargin -1in 

begin{table} % 
    centering
    caption{Obtained marks.} 
    begin{tabular}{p{16cm} l l l l }
        hline Name & Math & Phy & Chem & man 
        hline Salah & 80 & 68 & 60 & 57
        hline Muhammed & 72 & 62 & 66 & 63
        hline Ahmad & 75 & 70 & 71 & 69
        hline
    end{tabular}   
end{table}
end{document}