An accounting professor is notorious for being stingy in giving out good letter

Question

An accounting professor is notorious for being stingy in giving out good letter grades. In a large section of 120 students in the fall semester, she gave out only 6% As, 22% Bs, 38% Cs, and 34% Ds and Fs. Assuming that this was a representative class, compute the 95% confidence interval of the probability of getting at least a B from this professor. Use Table 1. (Round intermediate calculations to 4 decimal places. Round "z-value" and final answers to 3 decimal places.) Confidence interval ____ to ____

Explanation / Answer

here proportion of getting a least a B p=0.06+0.22=0.28

n=120

hence std error =(p(1-p)/n)1/2 =0.041

for 95% CI; z=1.96

therefore 95% confidence interval =estimate proportion -/+ z*std error =0.28 -/+1.96*0.041 =0.200 ; 0.360

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