# Thread: Normal Distribution

1. ## Normal Distribution

I found the answers to this question on this website (TUTORIAL SHEET 1 ? MA4704)

They are very different from my answers (it's an even problem, I can't check the back of the book). Can anyone walk me through one of a or b, and one of c or d?

Assume that X is normally distributed with a mean of 6 and a standard deviation of 3. Determine the value for x that solves each of the following.
a) P(X>x) = 0.5
Other site's answer: x=7.5

b) P(X>x) = 0.95
Other site's answer: x=8.85

c) P(x<X<9) = 0.2
Other site's answer: x=7.08

d) P(3<X<x) = 0.8
Other site's answer: x=7.9

2. Originally Posted by angel.white
I found the answers to this question on this website (TUTORIAL SHEET 1 ? MA4704)

They are very different from my answers (it's an even problem, I can't check the back of the book). Can anyone walk me through one of a or b, and one of c or d?

Assume that X is normally distributed with a mean of 6 and a standard deviation of 3. Determine the value for x that solves each of the following.
a) P(X>x) = 0.5
My answer: x=6 Mr F says: Correct.
Other site's answer: x=7.5 Mr F says: Wrong.

b) P(X>x) = 0.95
My answer: x=10.95 Mr F says: Correct.
Other site's answer: x=8.85 Mr F says: Wrong.

c) P(x<X<9) = 0.2
My answer: x=7.095 Mr F says: Correct.
Other site's answer: x=7.08 Mr F says: Correct.

The issue here is one of accuracy. A more correct answer is somewhere between these two answers.

d) P(3<X<x) = 0.8
My answer: x=11.235 Mr F says: Correct.
Other site's answer: x=7.9 Mr F says: Wrong.
..

3. Thank you, Mr. F. My imprecision is because I am getting numbers off of a look up table in the Appendix of my book (is that even math?!), but it only goes to 3 digits, so I go halfway between the two closest.

4. Originally Posted by angel.white
Thank you, Mr. F. My imprecision is because I am getting numbers off of a look up table in the Appendix of my book (is that even math?!), but it only goes to 3 digits, so I go halfway between the two closest.
Yes, they should be more digits. I'll see about finding my table (not like I made it, the one I just used, made by my lecturer) for you...

Here we go, this one should be much more accurate.