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 240 students in the fall semester, she gave out only 3% As, 18% Bs, 34% Cs, and 45% Ds and Fs. Assuming that this was a representative class, compute a 99% confidence interval of the probability of getting at least a B from this professor. Use Table 1. (Round intermediate calculations to 4 decimal places, "z" value to 2 decimal places, and final answers to 3 decimal places.) Confidence interval

Explanation / Answer

Answer to the question)

At leats a B implies either it can be B or A

Total percent of B and A = 3% and 18% = 21% ~ 0.21

Thus p = 0.21

n = 240

.

We need to find the 99% confidence intevral

Z value for 99% confidence interval is 2.58

.

Formula of Confidence interval is:

p - z*SE , p + z*SE

.

p = 0.21

z = 2.58

SE = sqrt(p*(1-p)/n)

SE = sqrt(0.21*0.79/240)

SE = sqrt(0.00069125)

SE = 0.02629

.

On plugging the values we get :

0.21 -2.58 *0.02629 , 0.21 +2.58*0.02629

0.1422 , 0.2778

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