# normal distribution with a mean and standard deviation...I am lost

• Oct 24th 2010, 08:01 AM
caligirl350
normal distribution with a mean and standard deviation...I am lost
X has a normal distribution with a mean of 80.0 and a standard deviation of 3.5. Find the following probabilities:
(A) P(x < 77.0)
(B) P(75.0 < x < 85.0)
(C) P(x > 85.0)
• Oct 24th 2010, 08:12 AM
Unknown008
A)

Find the z value fot x = 77.

$z = \dfrac{x - \mu}{\sigma} = \dfrac{77 - 80}{3.5} = -0.857$

So, you have
$P(X < 77) = P(z < -0.857)$

Look up your z table for the corresponding probability.

Do the same for the others.

B) $P(75.0

C) $P(X > 85.0) = P(z > 1.429)$

Post what you get! (Happy)
• Oct 24th 2010, 11:52 AM
mr fantastic
Quote:

Originally Posted by caligirl350
X has a normal distribution with a mean of 80.0 and a standard deviation of 3.5. Find the following probabilities:
(A) P(x < 77.0)
(B) P(75.0 < x < 85.0)
(C) P(x > 85.0)

Sorry, but these are very routine questions - I suggest you go to your class notes or textbook and review the examples that are certain to be there.