The length of human pregnancies from conception to birth varies according to a distribution that is approximately normal with mean 266 days and standard deviation 16 days.

a) What percent of pregnancies last less than 240 days (about 8 months)?

b) What percent of pregnancies last between 240 and 270 days (roughly between 8 and 9 months)

c) How long do the longest 20 % of pregnancies last?

Solution: (a) Here we need to compute

which means that 5.2081% of pregnancies last less than 240 days.

(b) Now we need

This implies that 54.6625% of pregnancies last between 240 and 270 days.

(c) We need to find C such that

This means that the longest 20 % of pregnancies last 279.4659 or more.

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