In a mid-size company, the distribution of the number of phone calls answered each day by each of the 12 receptionists is bell-shaped and has a mean of 48 and a standard deviation of 9. Using the Standard Deviation Rule (as presented in the book), what is the approximate percentage of daily phone calls numbering between 30 and 66?

95.44%

Step-by-step explanation:

Mean (m) = 48

Standard deviation (sd) = 9

n = 12

Zscore = (x - m) /sd

X = 30

Zscore = (30 - 48) / 9

Zscore = - 2

X = 66

Zscore = (66 - 48) / 9

Zscore = 2

percentage of daily phone calls numbering between 30 and 66 = P(z < 2) - p(z < - 2)

P(z < 2) = 0.9772

P(z < - 2) = 0.0228

Hence,

0.9772 - 0.0228 = 0.9544

0.9544 * 100% = 95.44

x² + 11x + 18 < 0  

[-11 ± sqrt(121 - 72)]/2 = (-11 ± 7)/2  

solution: (-9, -2)

step-by-step explanation: