I've tried so many things but I never get the correct answer. Here's an example:
For Find using a normal approximation
This is what I do:
But at this point, I get in a mess.
The correct answer is 0.9596
Thanks in advance for any replies.
I suggest you apply a "correction for continuity", so instead of finding
P(3 < Y < 13),
P(2.5 < Y < 13.5),
where Y has a Normal distribution with mean 8 and standard deviation 2.19.
This leads to
P(-2.51 < Z < 2.51)
where Z has a Normal(0,1) distribution, yielding a probability of 0.988.
(Yes, I know, that's not the answer in the book. Sometimes the book has the wrong answer.)