# Thread: Finding X Given Normal Probability

1. ## 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

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?

2. ## 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.

3. ## Re: Finding X Given Normal Probability

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?