=+32. 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-dayold specimens (“A Study of Twenty-Five-Year-Old Pulverized Fuel Ash Concrete Used in Foundation Structures,” Proc. Inst. Civil Engrs., 1985: 149–165):

7-day old: n1 5 68 x1 5 26.99 s1 5 4.89 28-day old: n2 5 74 x2 5 35.76 s2 5 6.43