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The lengths of pregnancies are normally distributed with a mean of 268 days and a standard deviation of 15 days.
a. find the probability of a pregnancy lasting 309 days or longer.
b. if the length of pregnancy is in the lowest 44​%, then the baby is premature. find the length that separates premature babies from those who are not premature.

Respuesta :

CastleRook
Let the lengths of pregnancies be X
X follows normal distribution with mean 268 and standard deviation 15 days
z=(X-269)/15

a. P(X>308)
z=(308-269)/15=2.6
thus:
P(X>308)=P(z>2.6)
=1-0.995
=0.005

b] Given that if the length of pregnancy is in lowest is 44%, then the baby is premature. We need to find the length that separates the premature babies from those who are not premature.
P(X<x)=0.44
P(Z<z)=0.44
z=-0.15
thus the value of x will be found as follows:
-0.05=(x-269)/15
-0.05(15)=x-269
-0.75=x-269
x=-0.75+269
x=268.78
The length that separates premature babies from those who are not premature is 268.78 days

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