I apologise for putting this in GC instead of Help, but I've been studying for my statistics exam for three hours now and I think there's one thing that's holding me back from getting an A on the exam.

When calculating the percentile rank I'm just not getting how to get the <i>n<sub>b</sub></i> variable. In the problems in the book we're the full distribution table, but on the exam we'll just have the mean, the standard deviation, and the total number of scores.

Anyone know how to derive <i>n<sub>b</sub></i> with the mean, standard deviation, and total cases?

Just for referrence, here's the formula for finding the percentile rank: P = (<i>n<sub>w</sub></i>[X-L] + <i>in<sub>b</sub></i>)<span style="font-size: 12.0pt; font-family: Symbol">¸ </span><span style="font-size: 12.0pt">N<i>i</i></span>

P = percentile rank
X = percentile point
L = lower real limit of X
<i>n<sub>w</sub></i> = number of scores within the score value
<i>n<sub>b</sub></i> = number of scores below X
N = total number of scores
i = 1

In this example, X = 65, L = 64.5, I'm assuming <i>n<sub>w</sub></i> = 1, N = 30, the standard deviation is 8, and the mean is 50.

So, so far I have P = (1[65 - 64.5] + 1<i>n<sub>b</sub></i>) <span style="font-size: 12.0pt; font-family: Symbol">¸</span> 30

I hate math soooo much. ;_;