Hi 'Standard Distribution': µ = 78.5 and σ = 13.6 n = 2680% confidence interval: z = 1.282 | Invnorm(.80 + .2/2) = Invnorm(.90) = 3.419 78.5 - 3.419 < 78.5 + 3.419 75.351< < 81.649 90% confident, interval will contain true population mean Wish You the Best in your Studies.