# Thread: Normal Probability Distribition

1. ## Normal Probability Distribition

So I got this problem wrong and again, I don't know why. If anyone can show me the right way to do it, I would be very appreciative.

The waiting time x at a fast-food restaurant during lunch time is approximately normally distributed with a mean of 4.5 min and a standard deviation of 1.2 min.

Find the value of the 75th percentile for x (in minutes).

I came up with 0.67 minutes and it was wrong. What I did was I drew my graph. I shaded in 75% (50% on the left and 25% on the right). I then looked up in the chart the closes z number to 0.2500 and got 0.67. I am not sure what I did wrong.

2. Originally Posted by ezwind72
So I got this problem wrong and again, I don't know why. If anyone can show me the right way to do it, I would be very appreciative.

The waiting time x at a fast-food restaurant during lunch time is approximately normally distributed with a mean of 4.5 min and a standard deviation of 1.2 min.

Find the value of the 75th percentile for x (in minutes).

I came up with 0.67 minutes and it was wrong. What I did was I drew my graph. I shaded in 75% (50% on the left and 25% on the right). I then looked up in the chart the closes z number to 0.2500 and got 0.67. I am not sure what I did wrong.
0.67 is the value for a Normal distribution with mean 0 and standard deviation 1. You need to interpret this in the context of your problem-- Normal with mean 4.5 and standard deviation 1.2.

3. OK, so then I would use the 0.67 and plug it into the z formula, correct? So I would have:

(x-4.5)/1.2 = 0.67

x-4.5 = 0.804

x = 5.304

Does that sound correct, or am I way off base?

4. Right!

5. Yes! Thank you so very much!! It feels good to understand!