Calculate the volumes of $0\mathrm{.}4$ M NaH$2$PO$4$ and $0\mathrm{.}2$ M Na$2$HPO$4$ necessary to make $1300$ mL of

buffer solution, having pH $=7\mathrm{.}0.$ pKa $=7\mathrm{.}20$

$($Answers given: V$($salt$)=725$ mL; V$($acid$)=525$ mL$).$ PLease kindly explain how to solve for the volumes, not only by showing the equation, please kindly explain stepwise how to calculate and get the different "answers" ie numbers in the qquations needed and how to solve for those numbers. Very grateful for your help!!