Question 1197534
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Hi  

'Standard Distribution': µ = 78.5 and σ = 13.6
n = 26
 {{{ME = z*sigma/sqrt(n)}}}
 80% confidence interval:  z = 1.282  |  Invnorm(.80 + .2/2) = Invnorm(.90) 
  {{{ME =1.282*13.6/sqrt(26)}}} = 3.419
78.5 - 3.419 {{{mu}}} < 78.5 + 3.419
75.351< {{{mu}}} < 81.649

90% confident,  interval will contain true population mean

Wish You the Best in your Studies.
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