Question

Assume that the mean length of a human pregnancy is 268 days with a standard deviation of 9 days. What percentage of human pregnancies do we expect to last no more than 272 days

Answer 1

Μ = 268 days, the population mean
σ = 9 days, the population standard deviation

For the random variable x = 272 days, the z-score is
z = (x-μ)/σ
   = (272 - 268)/9
   = 0.4444

From z-tables, obtain
P(z <= 0.6716) = 0.6716 = 67.2%

Answer:  67.2%