The length of human pregnancies from conception to birth var

The length of human pregnancies from conception to birth varies according to a normal distribution with mean 266 days and standard deviation 16 days. What percentage of pregnancies last longer than 275 days?

19.3%

22.9%

28.7%

30.5%

Solution

mean = 266

std dev = 16

for p(x =275) =

For x = 275, z = (275 - 266) /16 = 0.56

Hence P(x > 275) = P(z > 0.56) = [total area] - [area to the left of -0.56]

= 1 - 0.7123 = 0.287

= 28.7 %

hence option c is correct