Use the accompanying data to estimate with a 95%confidence interval the difference between true average compressive strength(N/mm² ) for 7-day-old concrete specimens and true average strength for 28-day-old specimens (“

A study of Twenty-Five-Year Old Pulverized Fuel Ash Concrete Used in Foundation Structures," Proc. Inst. Civil Engrs.,1985:149-165): \text { 7-day-old: } n_{1}=68 \bar{x}_{1}=26.99 s_{1}=4.89 \text { 28-day-old specimens: } n_{\mathbf{z}}=74 \bar{x}_{\mathbf{2}}=35.76 \mathrm{~s}_{\mathbf{Z}}=6.43