P$8-17$ Total, nondiversifiable, and diversifiable risk David Talbot randomly sclected securities from all those listed on the New York Stock Exchange for his portfolio. He began with a single security and added securities one by one until a total of $20$ securities were held in the portfolio. After each security was added, David calculated the portfolio standard deviation, ${}_{r_i}.$ The calculated values are shown in the following table.

$\backslash$table$[[\backslash$table$[[$Number of$],[$securitics$]],\backslash$table$[[$Portifolio$],[$risk$,{}_r,]],\backslash$table$[[$Number of$],[$sccuritis$]],\backslash$table$[[$Portfolio$],[$risk$,{}_r$