The accompanying table gives summary data on cube compressive strength (N/mm2) for concr...

The accompanying table gives summary data on cube compressive strength (N/mm²) for concrete specimens made with a pulverized fuel-ash mix (“A Study of Twenty-Five- Year-Old Pulverized Fuel Ash Concrete Used in Foundation Structures,” Proc. Inst. Civ. Engrs., Mar. 1985: 149–165):

Calculate and interpret a 99% CI for the difference between true average 7-day strength and true average 28-day strength.