The article "Direct Strut$-$and$-$Tie Model for Prestressed Deep Beams" presents measurements of the nominal shear strength $($in $kN)$ for a sample of $20$ prestressed concrete beams. The results are:

$\backslash$table$[[580,400,428,825,850,875,920,550,488,370],[575,750,636,360,430,590,735,950,420,465]]$

Assume that based on a very large number of previous measurements of other beams, the population of shear strengths is known to be approximately normal, with standard deviation, $=230kN.$ Find a $95\%$ confidence interval for the mean shear strength.