Finding X Given Normal Probability

• Jul 5th 2012, 09:56 AM
boba7523
Finding X Given Normal Probability
Hello everyone,

I have a question on a normal distribution problem where I have to find X given a certain probability. I know how to do it, but it seems like the method I used to achieve the answer is incorrect, and I don't understand why. I hope you guys can point me to the right direction (Hi)

Attachment 24232

I have posted an image with the problem above and my way of solving it vs. professor's way.

What I'm confused about: Why is this a left hand tail problem instead of a right hand tail like my way of doing it? Does the problem give out a hint stating that it's a left hand tail?
• Jul 5th 2012, 02:03 PM
pickslides
Re: Finding X Given Normal Probability
98% last longer than X hours, therefore only 2% have a shorter life.
The way you have done it is the opposite way around i.e.only 2% last longer than X hours, therefore 98% have a shorter life.
• Jul 8th 2012, 12:07 PM
boba7523
Re: Finding X Given Normal Probability
Quote:

Originally Posted by pickslides
98% last longer than X hours, therefore only 2% have a shorter life.
The way you have done it is the opposite way around i.e.only 2% last longer than X hours, therefore 98% have a shorter life.

How do i know if it's left hand tail or a right hand tail?